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Papers by Peggy Van Der Pol

Research paper thumbnail of Marijuana Motives Measure--Dutch Extended Version

Research paper thumbnail of Subject Index

Research paper thumbnail of Three-Year Course of Cannabis Dependence and Prediction of Persistence

European addiction research, Jan 28, 2015

To examine the course and the predictors of the persistence of cannabis dependence. Through canna... more To examine the course and the predictors of the persistence of cannabis dependence. Through cannabis outlets and chain referral, a prospective enriched community cohort of 207 young adults (aged 18-30) with DSM-IV cannabis dependence at baseline (T0) was formed and followed-up after 1.5 (T1) and 3 (T2) years. The presence of cannabis dependence, cannabis-related problems, functional impairment and treatment was assessed using the Composite International Diagnostic Interview (CIDI 3.0) and the Sheehan Disability Scale (SDS). Predictors of persistence were lifetime cannabis abuse and dependence symptoms, cannabis use characteristics, distant vulnerability factors (e.g. childhood adversity, family history of psychological/substance use problems, impulsivity, mental disorders), and proximal stress factors (recent life events, social support). Four groups were distinguished: persistent dependent (DDD: 28.0%), stable non-persistent (DNN: 40.6%), late non-persistent (DDN: 17.9%) and recurr...

Research paper thumbnail of The missing=smoking assumption: A fallacy in internet-based smoking cessation trials?

Nicotine & tobacco research : official journal of the Society for Research on Nicotine and Tobacco, Jan 5, 2015

Introduction In this study, penalized imputation (PI), a common approach to handle missing smokin... more Introduction In this study, penalized imputation (PI), a common approach to handle missing smoking status data and sometimes referred to as "missing=smoking", is compared to other missing data approaches using data from internet-based cessation trials. Two hypotheses were tested: 1.PI leads to more conservative effect estimates than complete observations analysis; and 2.PI and multiple imputation (MI) lead to similar effect estimates under balanced (equal missingness proportions among the trial arms) and unbalanced missingness. Methods First, the outcomes of 22 trials included in a recent Cochrane review on internet-based smoking cessation interventions were re-analysed using only the complete observations, and after applying PI. Second, in a simulation study outcomes under PI, complete observations analysis, and two types of MI were compared. For this purpose, individual patient data from one of the Cochrane review trials were used. Results of the missing data approaches ...

Research paper thumbnail of Grenzen aan gedogen

Verslaving, 2015

ABSTRACT Abstract [article in Dutch] In 1976 the change of the Opium Act in the Netherlands alrea... more ABSTRACT Abstract [article in Dutch] In 1976 the change of the Opium Act in the Netherlands already took into account the content of tetrahydrocannabinol (THC) in cannabis by placing hemp oil in Schedule I. In 2011 a recommendation followed for further tightening the classification based on the THC content. In 2013 a bill was proposed stipulating that all cannabis products of 15 % or more THC should be classified in Schedule I, containing psychoactive substances with an unacceptable risk (hard drugs). Hashish and marijuana with less than 15 % THC remain in Schedule II. The purpose of this measure is twofold: reducing the public health risks and – by increasing criminal risk – to reduce illegal cultivation and export of cannabis. In this article we focus on the public health aspects and describe the assumptions underlying this ‘15 % THC measure’ and to what extent they are supported by scientific research.

Research paper thumbnail of Reliability and validity of the Marijuana Motives Measure among young adult frequent cannabis users and associations with cannabis dependence

Addictive Behaviors, 2015

Factor analysis of the MMM confirmed a 5-factor structure.

Research paper thumbnail of The Role of Study and Work in Cannabis Use and Dependence Trajectories among Young Adult Frequent Cannabis Users

Life course theory considers events in study and work as potential turning points in deviance, in... more Life course theory considers events in study and work as potential turning points in deviance, including illicit drug use. This qualitative study explores the role of occupational life in cannabis use and dependence in young adults. Two and three years after the initial structured interview, 47 at baseline frequent cannabis users were interviewed in-depth about the dynamics underlying changes in their cannabis use and dependence. Overall, cannabis use and dependence declined, including interviewees who quit using cannabis completely, in particular with students, both during their study and after they got employed. Life course theory appeared to be a useful framework to explore how and why occupational life is related to cannabis use and dependence over time. Our study showed that life events in this realm are rather common in young adults and can have a strong impact on cannabis use. While sometimes changes in use are temporary, turning points can evolve from changes in educational and employment situations; an effect that seems to be related to the consequences of these changes in terms of amount of leisure time and agency (i.e., feelings of being in control).

Research paper thumbnail of Cannabis Dependence and Peer Selection in Social Networks of Frequent Users

SSRN Electronic Journal, 2000

In a Dutch longitudinal study on the dynamics of cannabis dependence, at baseline 600 frequent ca... more In a Dutch longitudinal study on the dynamics of cannabis dependence, at baseline 600 frequent cannabis users (≥ 3 days cannabis use per week in past 12 months) aged 18-30 years were interviewed. Nearly half of all participants (42%) met DSM-IV criteria for cannabis dependence in the 12 months prior to the interview. Participants were recruited by respondent-driven sampling; referrals

Research paper thumbnail of Reference values of exhaled nitric oxide in healthy children 1-5 years using off-line tidal breathing

Pediatric Pulmonology, 2014

Measurement of the fraction of exhaled nitric oxide (FeNO) is a useful test to diagnose and/or mo... more Measurement of the fraction of exhaled nitric oxide (FeNO) is a useful test to diagnose and/or monitor eosinophilic airway inflammation. The off-line tidal breathing method is used for measurements in young children, but reference values for preschool children are scarce. The objective of this study was to establish reference values for FeNO in healthy children 1-5 years old. We included 51 healthy children (23 males), mean age 32.5 months, from the general population and measured FeNO, using an off-line tidal breathing method with a chemiluminescence analyzer. The method proved feasible in 100% of the children. Geometric mean FeNO was 7.1 parts per billion (ppb), 95% confidence interval 2.8-11.5 ppb, with the 95th percentile 22.6 ppb. No significant difference was found between boys and girls, and no correlations were observed between FeNO and age, height, or weight. This study demonstrates that the off-line tidal breathing method is feasible to measure FeNO in preschool children and provides reference values of FeNO in healthy children 1-5 years of age.

Research paper thumbnail of Reliability and validity of the Severity of Dependence Scale for detecting cannabis dependence in frequent cannabis users

International Journal of Methods in Psychiatric Research, 2013

The Severity of Dependence Scale (SDS) measures with five items the degree of psychological depen... more The Severity of Dependence Scale (SDS) measures with five items the degree of psychological dependence on several illicit drugs, including cannabis. Its psychometric properties have not yet been examined in young adult frequent cannabis users, an eminently high-risk group for cannabis dependence.

Research paper thumbnail of The Dutch Cannabis Dependence (CanDep) study on the course of frequent cannabis use and dependence: objectives, methods and sample characteristics

International Journal of Methods in Psychiatric Research, 2011

This paper presents an overview of the prospective cohort design of the Dutch Cannabis Dependence... more This paper presents an overview of the prospective cohort design of the Dutch Cannabis Dependence (CanDep) study, which investigates (i) the three-year natural course of frequent cannabis use (≥ three days per week in the past 12 months) and cannabis dependence; and (ii) the factors involved in the transition from frequent non-dependent cannabis use to cannabis dependence, and remission from dependence. Besides its scientific relevance, this knowledge may contribute to improve selective and indicated prevention, early detection, treatment and cannabis policies. The secondary objectives are the identification of factors related to treatment seeking and the validation of self report measures of cannabis use.

Research paper thumbnail of Predicting the transition from frequent cannabis use to cannabis dependence: A three-year prospective study

Drug and Alcohol Dependence, 2013

Background: Frequent cannabis users are at high risk of dependence, still most (near) daily users... more Background: Frequent cannabis users are at high risk of dependence, still most (near) daily users are not dependent. It is unknown why some frequent users develop dependence, whereas others do not. This study aims to identify predictors of first-incidence DSM-IV cannabis dependence in frequent cannabis users. Methods: A prospective cohort of frequent cannabis users (aged 18-30, n = 600) with baseline and two follow-up assessments (18 and 36 months) was used. Only participants without lifetime diagnosis of DSM-IV cannabis dependence at baseline (n = 269) were selected. Incidence of DSM-IV cannabis dependence was established using the Composite International Diagnostic Interview version 3.0. Variables assessed as potential predictors of the development of cannabis dependence included sociodemographic factors, cannabis use variables (e.g., motives, consumption habits, cannabis exposure), vulnerability factors (e.g., childhood adversity, family history of mental disorders or substance use problems, personality, mental disorders), and stress factors (e.g., life events, social support). Results: Three-year cumulative incidence of cannabis dependence was 37.2% (95% CI = 30.7-43.8%). Independent predictors of the first incidence of cannabis dependence included: living alone, coping motives for cannabis use, number and type of recent negative life events (major financial problems), and number and type of cannabis use disorder symptoms (impaired control over use). Cannabis exposure variables and stable vulnerability factors did not independently predict first incidence of cannabis dependence. Conclusions: In a high risk population of young adult frequent cannabis users, current problems are more important predictors of first incidence cannabis dependence than the level and type of cannabis exposure and stable vulnerability factors.

Research paper thumbnail of Cross-sectional and prospective relation of cannabis potency, dosing and smoking behaviour with cannabis dependence: an ecological study

Addiction, 2014

Increased delta-9-tetrahydrocannabinol (THC) concentrations in cannabis may lead to higher THC ex... more Increased delta-9-tetrahydrocannabinol (THC) concentrations in cannabis may lead to higher THC exposure, cannabis dependence and treatment need, but users may also adapt the actual intake of THC through reduced inhalation of THC containing smoke (titration). We investigated whether consumers of stronger cannabis use less cannabis per joint or inhale less smoke than those using less potent cannabis and whether these factors predict cannabis dependence severity. Methods Heavy cannabis users (n = 98) brought their own cannabis, rolled a joint and smoked it ad libitum in a naturalistic setting. We analysed the content of the joint, its association with smoking behaviour and the cross-sectional and prospective (1.5-year follow-up) relations between smoking behaviour and cannabis dependence severity (total number of DSM-IV dependence symptoms). Results THC concentration in cannabis (range 1.10-24.70%) was correlated positively with cannabis dose per joint (b = 0.008, P = 0.01), but the resulting THC concentration per joint (range 0.24-15.72%) was associated negatively with inhalation volume (b = −0.05, P = 0.03). Smoking behaviour measures (number of puffs, inhaled volume, reduction of puff volume and puff duration while smoking) predicted follow-up dependence severity, independently of baseline dependence severity and monthly THC dose (number of joints × cannabis dose × cannabis THC concentration). Monthly THC dose only predicted follow-up dependence severity when unadjusted for baseline severity. Conclusions Cannabis users titrate their delta-9-tetrahydrocannabinol intake by inhaling lower volumes of smoke when smoking strong joints, but this does not fully compensate for the higher cannabis doses per joint when using strong cannabis. Thus, users of more potent cannabis are generally exposed to more delta-9-tetrahydrocannabinol. Smoking behaviour appears to be a stronger predictor for cannabis dependence severity than monthly delta-9-tetrahydrocannabinol dose.

Research paper thumbnail of Mental health differences between frequent cannabis users with and without dependence and the general population

Addiction, 2013

To compare the prevalence of mental disorders between frequent cannabis users with and without de... more To compare the prevalence of mental disorders between frequent cannabis users with and without dependence and the general population. Cross-sectional comparison of interview data. Enriched community sample of frequent cannabis users and a representative sample of non-users and non-frequent users from the general population. A total of 521 young adult (aged 18-30 years) frequent cannabis users, 252 of whom were with DSM-IV cannabis dependence (D+) and 269 without DSM-IV cannabis dependence (D-), and 1072 young adults from the general population. Multinomial logistic regression was used to compare groups regarding the presence of DSM-IV mental disorders. Detailed measures of cannabis use, childhood adversity and other substance use were considered confounders. Compared with the general population, externalizing disorders were more prevalent in D- [odds ratio (OR) = 8.91, P < 0.001] and most prevalent in D+ (OR = 17.75, P < 0.001), but internalizing disorders were associated only with D+ (mood OR = 4.15, P < 0.001; anxiety OR = 2.20, P = 0.002). Associations were attenuated (and often became non-significant) after correction for childhood adversity and substance use other than cannabis. However, the prevalence of mental disorders remained higher in D+ compared with D- (OR = 2.40, P < 0.001), although cannabis use patterns were remarkably similar. Cannabis use patterns, childhood adversity and the use of other substances are similar in dependent and non-dependent frequent cannabis users. With the exception of more externalizing disorders, the mental health condition of non-dependent frequent cannabis users is similar to that of the general population, whereas it is worse in dependent frequent cannabis users.

Research paper thumbnail of Validation of self-reported cannabis dose and potency: an ecological study

Addiction, 2013

To assess the reliability and validity of self-reported cannabis dose and potency measures. Cross... more To assess the reliability and validity of self-reported cannabis dose and potency measures. Cross-sectional study comparing self-reports with objective measures of amount of cannabis and delta-9-tetrahydrocannabinol (THC) concentration. Ecological study with assessments at participants' homes or in a coffee shop. Young adult frequent cannabis users (n = 106) from the Dutch Cannabis Dependence (CanDep) study. The objectively measured amount of cannabis per joint (dose in grams) was compared with self-reported estimates using a prompt card and average number of joints made from 1 g of cannabis. In addition, objectively assessed THC concentration in the participant's cannabis was compared with self-reported level of intoxication, subjective estimate of cannabis potency and price per gram of cannabis. Objective estimates of doses per joint (0.07-0.88 g/joint) and cannabis potency (1.1-24.7%) varied widely. Self-reported measures of dose were imprecise, but at group level, average dose per joint was estimated accurately with the number of joints made from 1 g [limit of agreement (LOA) = -0.02 g, 95% confidence interval (CI) = -0.29; 0.26], whereas the prompt card resulted in serious underestimation (LOA = 0.14 g, 95% CI = -0.10; 0.37). THC concentration in cannabis was associated with subjective potency ['average' 3.77% (P = 0.002) and '(very) strong' 5.13% more THC (P < 0.001) than '(very) mild' cannabis] and with cannabis price (about 1% increase in THC concentration per euro spent on 1 g of cannabis, P < 0.001), but not with level of intoxication. Self-report measures relating to cannabis use appear at best to be associated weakly with objective measures. Of the self-report measures, number of joints per gram, cannabis price and subjective potency have at least some validity.

Research paper thumbnail of The role of leisure and delinquency in frequent cannabis use and dependence trajectories among young adults

The International journal on drug policy, 2015

The link between leisure and cannabis use has been widely studied, but less so for young adults, ... more The link between leisure and cannabis use has been widely studied, but less so for young adults, and rarely with a focus on frequent cannabis use. Also, little is known about how changes in leisure develop over time and how they are related to transitions in cannabis use and dependence. As part of a 3-year longitudinal project, in a qualitative study 47 frequent male and female young adult cannabis users with (n=23) and without (n=24) dependence at baseline were interviewed in-depth after 1.5 and 3 years. Frequent cannabis users (at baseline ≥3 days per week in the past 12 months) are involved in similar leisure activities as the general young adult population and live rather conventional lives, generally away from a delinquent subculture. They mostly regulate their cannabis use to leisure time, to enhance other leisure activities, including socialising and video gaming. While they often give precedence to responsibilities (e.g. work and study), dependent and non-dependent users dif...

Research paper thumbnail of Marijuana Motives Measure--Dutch Extended Version

Research paper thumbnail of Subject Index

Research paper thumbnail of Three-Year Course of Cannabis Dependence and Prediction of Persistence

European addiction research, Jan 28, 2015

To examine the course and the predictors of the persistence of cannabis dependence. Through canna... more To examine the course and the predictors of the persistence of cannabis dependence. Through cannabis outlets and chain referral, a prospective enriched community cohort of 207 young adults (aged 18-30) with DSM-IV cannabis dependence at baseline (T0) was formed and followed-up after 1.5 (T1) and 3 (T2) years. The presence of cannabis dependence, cannabis-related problems, functional impairment and treatment was assessed using the Composite International Diagnostic Interview (CIDI 3.0) and the Sheehan Disability Scale (SDS). Predictors of persistence were lifetime cannabis abuse and dependence symptoms, cannabis use characteristics, distant vulnerability factors (e.g. childhood adversity, family history of psychological/substance use problems, impulsivity, mental disorders), and proximal stress factors (recent life events, social support). Four groups were distinguished: persistent dependent (DDD: 28.0%), stable non-persistent (DNN: 40.6%), late non-persistent (DDN: 17.9%) and recurr...

Research paper thumbnail of The missing=smoking assumption: A fallacy in internet-based smoking cessation trials?

Nicotine & tobacco research : official journal of the Society for Research on Nicotine and Tobacco, Jan 5, 2015

Introduction In this study, penalized imputation (PI), a common approach to handle missing smokin... more Introduction In this study, penalized imputation (PI), a common approach to handle missing smoking status data and sometimes referred to as "missing=smoking", is compared to other missing data approaches using data from internet-based cessation trials. Two hypotheses were tested: 1.PI leads to more conservative effect estimates than complete observations analysis; and 2.PI and multiple imputation (MI) lead to similar effect estimates under balanced (equal missingness proportions among the trial arms) and unbalanced missingness. Methods First, the outcomes of 22 trials included in a recent Cochrane review on internet-based smoking cessation interventions were re-analysed using only the complete observations, and after applying PI. Second, in a simulation study outcomes under PI, complete observations analysis, and two types of MI were compared. For this purpose, individual patient data from one of the Cochrane review trials were used. Results of the missing data approaches ...

Research paper thumbnail of Grenzen aan gedogen

Verslaving, 2015

ABSTRACT Abstract [article in Dutch] In 1976 the change of the Opium Act in the Netherlands alrea... more ABSTRACT Abstract [article in Dutch] In 1976 the change of the Opium Act in the Netherlands already took into account the content of tetrahydrocannabinol (THC) in cannabis by placing hemp oil in Schedule I. In 2011 a recommendation followed for further tightening the classification based on the THC content. In 2013 a bill was proposed stipulating that all cannabis products of 15 % or more THC should be classified in Schedule I, containing psychoactive substances with an unacceptable risk (hard drugs). Hashish and marijuana with less than 15 % THC remain in Schedule II. The purpose of this measure is twofold: reducing the public health risks and – by increasing criminal risk – to reduce illegal cultivation and export of cannabis. In this article we focus on the public health aspects and describe the assumptions underlying this ‘15 % THC measure’ and to what extent they are supported by scientific research.

Research paper thumbnail of Reliability and validity of the Marijuana Motives Measure among young adult frequent cannabis users and associations with cannabis dependence

Addictive Behaviors, 2015

Factor analysis of the MMM confirmed a 5-factor structure.

Research paper thumbnail of The Role of Study and Work in Cannabis Use and Dependence Trajectories among Young Adult Frequent Cannabis Users

Life course theory considers events in study and work as potential turning points in deviance, in... more Life course theory considers events in study and work as potential turning points in deviance, including illicit drug use. This qualitative study explores the role of occupational life in cannabis use and dependence in young adults. Two and three years after the initial structured interview, 47 at baseline frequent cannabis users were interviewed in-depth about the dynamics underlying changes in their cannabis use and dependence. Overall, cannabis use and dependence declined, including interviewees who quit using cannabis completely, in particular with students, both during their study and after they got employed. Life course theory appeared to be a useful framework to explore how and why occupational life is related to cannabis use and dependence over time. Our study showed that life events in this realm are rather common in young adults and can have a strong impact on cannabis use. While sometimes changes in use are temporary, turning points can evolve from changes in educational and employment situations; an effect that seems to be related to the consequences of these changes in terms of amount of leisure time and agency (i.e., feelings of being in control).

Research paper thumbnail of Cannabis Dependence and Peer Selection in Social Networks of Frequent Users

SSRN Electronic Journal, 2000

In a Dutch longitudinal study on the dynamics of cannabis dependence, at baseline 600 frequent ca... more In a Dutch longitudinal study on the dynamics of cannabis dependence, at baseline 600 frequent cannabis users (≥ 3 days cannabis use per week in past 12 months) aged 18-30 years were interviewed. Nearly half of all participants (42%) met DSM-IV criteria for cannabis dependence in the 12 months prior to the interview. Participants were recruited by respondent-driven sampling; referrals

Research paper thumbnail of Reference values of exhaled nitric oxide in healthy children 1-5 years using off-line tidal breathing

Pediatric Pulmonology, 2014

Measurement of the fraction of exhaled nitric oxide (FeNO) is a useful test to diagnose and/or mo... more Measurement of the fraction of exhaled nitric oxide (FeNO) is a useful test to diagnose and/or monitor eosinophilic airway inflammation. The off-line tidal breathing method is used for measurements in young children, but reference values for preschool children are scarce. The objective of this study was to establish reference values for FeNO in healthy children 1-5 years old. We included 51 healthy children (23 males), mean age 32.5 months, from the general population and measured FeNO, using an off-line tidal breathing method with a chemiluminescence analyzer. The method proved feasible in 100% of the children. Geometric mean FeNO was 7.1 parts per billion (ppb), 95% confidence interval 2.8-11.5 ppb, with the 95th percentile 22.6 ppb. No significant difference was found between boys and girls, and no correlations were observed between FeNO and age, height, or weight. This study demonstrates that the off-line tidal breathing method is feasible to measure FeNO in preschool children and provides reference values of FeNO in healthy children 1-5 years of age.

Research paper thumbnail of Reliability and validity of the Severity of Dependence Scale for detecting cannabis dependence in frequent cannabis users

International Journal of Methods in Psychiatric Research, 2013

The Severity of Dependence Scale (SDS) measures with five items the degree of psychological depen... more The Severity of Dependence Scale (SDS) measures with five items the degree of psychological dependence on several illicit drugs, including cannabis. Its psychometric properties have not yet been examined in young adult frequent cannabis users, an eminently high-risk group for cannabis dependence.

Research paper thumbnail of The Dutch Cannabis Dependence (CanDep) study on the course of frequent cannabis use and dependence: objectives, methods and sample characteristics

International Journal of Methods in Psychiatric Research, 2011

This paper presents an overview of the prospective cohort design of the Dutch Cannabis Dependence... more This paper presents an overview of the prospective cohort design of the Dutch Cannabis Dependence (CanDep) study, which investigates (i) the three-year natural course of frequent cannabis use (≥ three days per week in the past 12 months) and cannabis dependence; and (ii) the factors involved in the transition from frequent non-dependent cannabis use to cannabis dependence, and remission from dependence. Besides its scientific relevance, this knowledge may contribute to improve selective and indicated prevention, early detection, treatment and cannabis policies. The secondary objectives are the identification of factors related to treatment seeking and the validation of self report measures of cannabis use.

Research paper thumbnail of Predicting the transition from frequent cannabis use to cannabis dependence: A three-year prospective study

Drug and Alcohol Dependence, 2013

Background: Frequent cannabis users are at high risk of dependence, still most (near) daily users... more Background: Frequent cannabis users are at high risk of dependence, still most (near) daily users are not dependent. It is unknown why some frequent users develop dependence, whereas others do not. This study aims to identify predictors of first-incidence DSM-IV cannabis dependence in frequent cannabis users. Methods: A prospective cohort of frequent cannabis users (aged 18-30, n = 600) with baseline and two follow-up assessments (18 and 36 months) was used. Only participants without lifetime diagnosis of DSM-IV cannabis dependence at baseline (n = 269) were selected. Incidence of DSM-IV cannabis dependence was established using the Composite International Diagnostic Interview version 3.0. Variables assessed as potential predictors of the development of cannabis dependence included sociodemographic factors, cannabis use variables (e.g., motives, consumption habits, cannabis exposure), vulnerability factors (e.g., childhood adversity, family history of mental disorders or substance use problems, personality, mental disorders), and stress factors (e.g., life events, social support). Results: Three-year cumulative incidence of cannabis dependence was 37.2% (95% CI = 30.7-43.8%). Independent predictors of the first incidence of cannabis dependence included: living alone, coping motives for cannabis use, number and type of recent negative life events (major financial problems), and number and type of cannabis use disorder symptoms (impaired control over use). Cannabis exposure variables and stable vulnerability factors did not independently predict first incidence of cannabis dependence. Conclusions: In a high risk population of young adult frequent cannabis users, current problems are more important predictors of first incidence cannabis dependence than the level and type of cannabis exposure and stable vulnerability factors.

Research paper thumbnail of Cross-sectional and prospective relation of cannabis potency, dosing and smoking behaviour with cannabis dependence: an ecological study

Addiction, 2014

Increased delta-9-tetrahydrocannabinol (THC) concentrations in cannabis may lead to higher THC ex... more Increased delta-9-tetrahydrocannabinol (THC) concentrations in cannabis may lead to higher THC exposure, cannabis dependence and treatment need, but users may also adapt the actual intake of THC through reduced inhalation of THC containing smoke (titration). We investigated whether consumers of stronger cannabis use less cannabis per joint or inhale less smoke than those using less potent cannabis and whether these factors predict cannabis dependence severity. Methods Heavy cannabis users (n = 98) brought their own cannabis, rolled a joint and smoked it ad libitum in a naturalistic setting. We analysed the content of the joint, its association with smoking behaviour and the cross-sectional and prospective (1.5-year follow-up) relations between smoking behaviour and cannabis dependence severity (total number of DSM-IV dependence symptoms). Results THC concentration in cannabis (range 1.10-24.70%) was correlated positively with cannabis dose per joint (b = 0.008, P = 0.01), but the resulting THC concentration per joint (range 0.24-15.72%) was associated negatively with inhalation volume (b = −0.05, P = 0.03). Smoking behaviour measures (number of puffs, inhaled volume, reduction of puff volume and puff duration while smoking) predicted follow-up dependence severity, independently of baseline dependence severity and monthly THC dose (number of joints × cannabis dose × cannabis THC concentration). Monthly THC dose only predicted follow-up dependence severity when unadjusted for baseline severity. Conclusions Cannabis users titrate their delta-9-tetrahydrocannabinol intake by inhaling lower volumes of smoke when smoking strong joints, but this does not fully compensate for the higher cannabis doses per joint when using strong cannabis. Thus, users of more potent cannabis are generally exposed to more delta-9-tetrahydrocannabinol. Smoking behaviour appears to be a stronger predictor for cannabis dependence severity than monthly delta-9-tetrahydrocannabinol dose.

Research paper thumbnail of Mental health differences between frequent cannabis users with and without dependence and the general population

Addiction, 2013

To compare the prevalence of mental disorders between frequent cannabis users with and without de... more To compare the prevalence of mental disorders between frequent cannabis users with and without dependence and the general population. Cross-sectional comparison of interview data. Enriched community sample of frequent cannabis users and a representative sample of non-users and non-frequent users from the general population. A total of 521 young adult (aged 18-30 years) frequent cannabis users, 252 of whom were with DSM-IV cannabis dependence (D+) and 269 without DSM-IV cannabis dependence (D-), and 1072 young adults from the general population. Multinomial logistic regression was used to compare groups regarding the presence of DSM-IV mental disorders. Detailed measures of cannabis use, childhood adversity and other substance use were considered confounders. Compared with the general population, externalizing disorders were more prevalent in D- [odds ratio (OR) = 8.91, P < 0.001] and most prevalent in D+ (OR = 17.75, P < 0.001), but internalizing disorders were associated only with D+ (mood OR = 4.15, P < 0.001; anxiety OR = 2.20, P = 0.002). Associations were attenuated (and often became non-significant) after correction for childhood adversity and substance use other than cannabis. However, the prevalence of mental disorders remained higher in D+ compared with D- (OR = 2.40, P < 0.001), although cannabis use patterns were remarkably similar. Cannabis use patterns, childhood adversity and the use of other substances are similar in dependent and non-dependent frequent cannabis users. With the exception of more externalizing disorders, the mental health condition of non-dependent frequent cannabis users is similar to that of the general population, whereas it is worse in dependent frequent cannabis users.

Research paper thumbnail of Validation of self-reported cannabis dose and potency: an ecological study

Addiction, 2013

To assess the reliability and validity of self-reported cannabis dose and potency measures. Cross... more To assess the reliability and validity of self-reported cannabis dose and potency measures. Cross-sectional study comparing self-reports with objective measures of amount of cannabis and delta-9-tetrahydrocannabinol (THC) concentration. Ecological study with assessments at participants' homes or in a coffee shop. Young adult frequent cannabis users (n = 106) from the Dutch Cannabis Dependence (CanDep) study. The objectively measured amount of cannabis per joint (dose in grams) was compared with self-reported estimates using a prompt card and average number of joints made from 1 g of cannabis. In addition, objectively assessed THC concentration in the participant's cannabis was compared with self-reported level of intoxication, subjective estimate of cannabis potency and price per gram of cannabis. Objective estimates of doses per joint (0.07-0.88 g/joint) and cannabis potency (1.1-24.7%) varied widely. Self-reported measures of dose were imprecise, but at group level, average dose per joint was estimated accurately with the number of joints made from 1 g [limit of agreement (LOA) = -0.02 g, 95% confidence interval (CI) = -0.29; 0.26], whereas the prompt card resulted in serious underestimation (LOA = 0.14 g, 95% CI = -0.10; 0.37). THC concentration in cannabis was associated with subjective potency ['average' 3.77% (P = 0.002) and '(very) strong' 5.13% more THC (P < 0.001) than '(very) mild' cannabis] and with cannabis price (about 1% increase in THC concentration per euro spent on 1 g of cannabis, P < 0.001), but not with level of intoxication. Self-report measures relating to cannabis use appear at best to be associated weakly with objective measures. Of the self-report measures, number of joints per gram, cannabis price and subjective potency have at least some validity.

Research paper thumbnail of The role of leisure and delinquency in frequent cannabis use and dependence trajectories among young adults

The International journal on drug policy, 2015

The link between leisure and cannabis use has been widely studied, but less so for young adults, ... more The link between leisure and cannabis use has been widely studied, but less so for young adults, and rarely with a focus on frequent cannabis use. Also, little is known about how changes in leisure develop over time and how they are related to transitions in cannabis use and dependence. As part of a 3-year longitudinal project, in a qualitative study 47 frequent male and female young adult cannabis users with (n=23) and without (n=24) dependence at baseline were interviewed in-depth after 1.5 and 3 years. Frequent cannabis users (at baseline ≥3 days per week in the past 12 months) are involved in similar leisure activities as the general young adult population and live rather conventional lives, generally away from a delinquent subculture. They mostly regulate their cannabis use to leisure time, to enhance other leisure activities, including socialising and video gaming. While they often give precedence to responsibilities (e.g. work and study), dependent and non-dependent users dif...