How Cannabis Users Obtain and Purchase Cannabis: A Comparison of Cannabis Users from European Countries with Different Cannabis Policies.

Objective: This study examines the role of cannabis policy in how cannabis users obtain and purchase cannabis. Methods: A survey was conducted in the Dutch coffeeshops among current cannabis users (n = 1255) aged 18-40 from seven European countries with different cannabis policies. This study investigated whether acquisition methods and supply sources were associated with national cannabis policy, controlling for gender, age, and frequency of use. Results: Cross-national differences notwithstanding, cannabis was easily available to current cannabis users in Europe. Within and across countries, users acquired cannabis in various ways and buyers purchased it from various sources, representing a mixture of open, closed, and semi-open retail markets. Buying cannabis was the most common method of acquisition. Among participants who reported buying their cannabis (n = 929), buying from friends was the most common source of supply, followed by street dealers, home dealers, and delivery services. The vast majority of Dutch participants reported buying cannabis from coffeeshops. Contrariwise, French buyers were more likely to buy cannabis from street dealers and delivery services, and Greek buyers to buy it from home dealers and friends. Overall, the Internet played a marginal role in purchasing cannabis. Conclusion: Our findings confirm the significant role of social supply across Europe. Although cross-national differences were rather common in cannabis acquisition and supply, yet they were not unidirectionally linked with the punitiveness of national cannabis policy. Findings suggest a differentiated normalization of the cannabis retail market, with users often preferring to buy cannabis in a regulated or legal market.

Why do people use new psychoactive substances? Development of a new measurement tool in six European countries.

INTRODUCTION: New psychoactive substances (NPS) pose a public health threat. Many studies have tried to identify the reasons of NPS use; however, none of them have so far used any standardised measures. The aim of this study was (i) to develop and cross-culturally validate the New Psychoactive Substance Use Motives Measure (NPSMM) and (ii) to compare motives of NPS use across countries and user types. METHODS: Three subgroups (socially marginalised users, nightlife attendees and members of online communities) of NPS users (N = 3023) were recruited from six EU member countries. Demographics, motives and types of NPS used were assessed. NPS use motives were measured by adapting the extended six-factor version of the Marijuana Motives Measure. RESULTS: Exploratory and confirmatory factor analysis resulted in a similar five-factor solution across most of the countries: coping, enhancement, social, conformity and expansion motives. Marginalised users scored higher on coping and conformity motives, nightlife groups showed higher endorsement of social motive, whereas online community users showed higher scores on expansion motives. Various types of NPS were also associated with different motives. CONCLUSION: NPS use motives might be associated with both the groups of users and the specific types of NPS being consumed. Expansion (psychedelics) and enhancement (stimulants) motives seemed to be linked to the chosen NPS product type, while coping, social and conformity motives were rather associated with user groups. NPSMM was found to be a valid instrument to measure NPS motives.

Short- and long-term effects of digital prevention and treatment interventions for cannabis use reduction: A systematic review and meta-analysis.

BACKGROUND: Frequent Cannabis use has been linked to a variety of negative mental, physical, and social consequences. We assessed the effects of digital prevention and treatment interventions on Cannabis use reduction in comparison with control conditions. METHODS: Systematic review with two separate meta-analyses. Thirty randomized controlled trials met the inclusion criteria for the review, and 21 were included in the meta-analyses. Primary outcome was self-reported Cannabis use at post-treatment and follow-up. Hedges's g was calculated for all comparisons with non-active control. Risk of bias was examined with the Cochrane risk-of-bias tool. RESULTS: The systematic review included 10 prevention interventions targeting 8138 participants (aged 12 to 20) and 20 treatment interventions targeting 5195 Cannabis users (aged 16 to 40). The meta-analyses showed significantly reduced Cannabis use at post-treatment in the prevention interventions (6 studies, N = 2564, g = 0.33; 95% CI 0.13 to 0.54, p = 0.001) and in the treatment interventions (17 comparisons, N = 3813, g = 0.12; 95% CI 0.02 to 0.22, p = 0.02) as compared with controls. The effects of prevention interventions were maintained at follow-ups of up to 12 months (5 comparisons, N = 2445, g = 0.22; 95% CI 0.12 to 0.33, p < 0.001) but were no longer statistically significant for treatment interventions. CONCLUSIONS: Digital prevention and treatment interventions showed small, significant reduction effects on Cannabis use in diverse target populations at post-treatment compared to controls. For prevention interventions, the post-treatment effects were maintained at follow-up up to 12 months later.

Health and Social Problems Associated with Recent Novel Psychoactive Substance (NPS) Use Amongst Marginalised, Nightlife and Online Users in Six European Countries.

Continued diversification and use of new psychoactive substances (NPS) across Europe remains a public health challenge. The study describes health and social consequences of recent NPS use as reported in a survey of marginalised, nightlife and online NPS users in the Netherlands, Hungary, Portugal, Ireland, Germany and Poland (n = 3023). Some respondents were unable to categorise NPS they had used. Use of 'herbal blends' and 'synthetic cannabinoids obtained pure' was most reported in Germany, Poland and Hungary, and use of 'branded stimulants' and 'stimulants/empathogens/nootropics obtained pure' was most reported in the Netherlands. Increased heart rate and palpitation, dizziness, anxiety, horror trips and headaches were most commonly reported acute side effects. Marginalised users reported substantially more acute side effects, more mid- and long-term mental and physical problems, and more social problems. Development of country-specific NPS awareness raising initiatives, health and social service needs assessments, and targeted responses are warranted.

An Exploratory Study of Cannabis Festivals and Their Attendees in Two European Cities: Amsterdam and Berlin.

This study was designed to explore the nature and features of cannabis festivals, characteristics of festival participants, and reasons for attendance. A field study in two European cities (Amsterdam and Berlin) included participant observation at the festivals, interviews with local organizers, and a survey among festival attendees (n = 728). Both festivals had common features, but also showed distinct differences. At both festivals, nine out of 10 participants were current, often daily cannabis users. Participants were mainly young adults (mean = 26.2 years), but younger in Berlin than in Amsterdam. Common reasons for festival attendance were "protest/activism" and "entertainment." Protest/activism was more likely in Berlin, among daily cannabis users, and participants aged 25+ years. Entertainment was more likely in Amsterdam, among non-daily cannabis users, and participants younger than 25 years. Although similar in political aim, cannabis festivals are characterized by distinctive local features, as well as differences in attendee profile and reasons for festival participation. Findings suggest that the latter differences are driven by differences in cannabis policy, with a stronger tendency towards protest/activism in countries with a less liberal, or more restrictive, cannabis policy. Future research should include more countries, representing a wider variation in cannabis policies.

Tightening the Dutch coffee shop policy: Evaluation of the private club and the residence criterion.

BACKGROUND: The Dutch coffee shop policy was tightened in 2012. Two additional criteria that coffee shops must adhere to in order for them to be tolerated came into force: the private club and the residence criterion. Coffee shops were only permitted to give access to members and only residents of the Netherlands were permitted to become a member. This tightened policy sought to make coffee shops smaller and more controllable, to reduce the nuisance associated with coffee shops and to reduce the number of foreign visitors attracted by the coffee shops. Enforcement began in the southern provinces. The private club criterion was abolished at the end of 2012. METHODS: A sample of fourteen municipalities with coffee shops was drawn. Seven in the south were treated as an 'experimental group' and the others as 'comparison group'. A baseline assessment and follow-ups at six and 18 months were performed. A combination of methods was applied: interviews with local experts, surveys with neighbourhood residents, coffee shop visitors and cannabis users, and ethnographic field work. RESULTS: Drugs tourism to coffee shops swiftly declined in 2012. The coffee shops also lost a large portion of their local customers, since users did not want to register as a member. The illegal market expanded. Neighbourhood residents experienced a greater amount of nuisance caused by dealer activities. After abolishment of the private club criterion, residents of the Netherlands largely returned to the coffee shops. Drug tourists still remained largely absent. Neighbourhood residents experienced more nuisance from coffee shops again. Illegal cannabis sale was tempered. No effect on cannabis use was found. CONCLUSION: The quick and robust shifts in the users' market in reaction to the policy changes illustrate the power of policy, but also the limitations caused by the dynamic and resilient nature of the Dutch cannabis supply market.

Exploring the Attractiveness of New Psychoactive Substances (NPS) among Experienced Drug Users.

A growing number of New Psychoactive Substances (NPS) appear yearly on the European market (81 for the first time in 2013, adding to a total of over 350 NPS). Using semi-structured interviews with 25 Dutch experienced recreational drug users, the role of the Internet and friends in gathering and exchanging information about NPS was elaborated. Furthermore, we investigated how NPS were acquired and which aspects make NPS more or less attractive, including their legal status. It appeared that the Internet was an important source of information about NPS in general. Personal experiences with NPS were preferably shared face-to-face with friends, as for privacy reasons users were cautious to post their experiences on web sites and forums. NPS were usually obtained or bought from friends or-to a lesser extent-purchased via the Internet. The preference for a specific NPS depended on the desired effects (mostly stimulant or psychedelic), price (similar to MDMA or amphetamine), duration of effect (preferably around four hours), and setting (at home, at festivals, or in nightlife). Legal status was not relevant for the decision to use NPS. Most NPS are not superior to the already marketed drugs, and do not displace conventional illicit drugs.

Three-Year Course of Cannabis Dependence and Prediction of Persistence.

AIMS: To examine the course and the predictors of the persistence of cannabis dependence. METHODS: 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). RESULTS: Four groups were distinguished: persistent dependent (DDD: 28.0%), stable non-persistent (DNN: 40.6%), late non-persistent (DDN: 17.9%) and recurrent dependent (DND: 13.5%). At T2, persisters (DDD) reported significantly more (heavy) cannabis use and cannabis problems than non-persisters (DNN/DDN/DND). Treatment seeking for cannabis-related problems was rare, even among persisters (15.5%). The number (OR = 1.23 (1.03-1.48)) and type ('role impairment' OR = 2.85 (1.11-7.31), 'use despite problems' OR = 2.34 (1.15-4.76)) of lifetime cannabis abuse/dependence symptoms were the only independent predictors of persistence with a total explained variance of 8.8%. CONCLUSIONS: Persistence of cannabis dependence in the community is low, difficult to predict, and associated with a negative outcome. The substantial proportion of stable non-persisters suggests that screening and monitoring or low-threshold brief interventions may suffice for many non-treatment-seeking cannabis-dependent people. However, those with many lifetime abuse/dependence symptoms may benefit from more intensive interventions.

Lessons from conducting trans-national Internet-mediated participatory research with hidden populations of cannabis cultivators.

BACKGROUND: Internet-mediated research methods are increasingly used to access hidden populations. The International Cannabis Cultivation Questionnaire (ICCQ) is an online survey designed to facilitate international comparisons into the relatively under-researched but increasingly significant phenomenon of domestic cannabis cultivation. The Global Cannabis Cultivation Research Consortium has used the ICCQ to survey over 6000 cannabis cultivators across 11 countries. In this paper, we describe and reflect upon our methodological approach, focusing on the digital and traditional recruitment methods used to access this hidden population and the challenges of working across multiple countries, cultures and languages. METHODS: Descriptive statistics showing eligibility and completion rates and recruitment source by country of residence. RESULTS: Over three quarters of eligible respondents who were presented with the survey were included in the final sample of n=6528. English-speaking countries expended more effort to recruit participants than non-English-speaking countries. The most effective recruitment modes were cannabis websites/groups (33%), Facebook (14%) and news articles (11%). While respondents recruited through news articles were older, growing practice variables were strikingly similar between these main recruitment modes. CONCLUSION: Through this process, we learnt that there are trade-offs between hosting multiple surveys in each country vs. using one integrated database. We also found that although perceived anonymity is routinely assumed to be a benefit of using digital research methodologies, there are significant limits to research participant anonymity in the current era of mass digital surveillance, especially when the target group is particularly concerned about evading law enforcement. Finally, we list a number of specific recommendations for future researchers utilising Internet-mediated approaches to researching hidden populations.

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

The Marijuana Motives Measure (MMM) has so far been examined mainly in student populations, often with relatively limited involvement in cannabis use. This study evaluated the factor structure of the MMM in a demographically mixed sample of 600 young adult (18-30 years) frequent (≥ 3 days per week) cannabis users in the Netherlands. Analysis confirmed a five-factor solution, denoting coping, enhancement, social, conformity and expansion motives. Additionally, the original MMM was extended with two items (boredom and habit), which formed a distinct, internally consistent sixth factor labelled routine motives. In a multivariable logistic regression analysis, coping and routine motives showed significant associations with 12-month DSM-IV cannabis dependence. The results suggest general reliability and validity of the MMM in a heterogeneous population of experienced cannabis users.

The role of leisure and delinquency in frequent cannabis use and dependence trajectories among young adults.

BACKGROUND: 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. METHOD: 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. RESULTS: 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 differed in whether they actively adapted their leisure activities to their cannabis use, or their cannabis use to their leisure time. Both types of and time spent on leisure activities were associated with transitions in use and dependence. CONCLUSIONS: While our findings generally support the normalisation thesis, it is questionable whether frequent but non-problematic cannabis use is socially accepted in wider society. This study also questions the diagnostic dependence vs. non-dependence dichotomy, and adds finer distinctions to the concept of cannabis dependence. Implications for prevention and treatment include facilitating structured spending of leisure time (e.g. sports), and targeting frequent users who spent much leisure time video gaming at home.

Criminal involvement and crime specialization among crack users in the Netherlands.

BACKGROUND/AIMS: Crack users in the Netherlands are an ageing and diverse population with longstanding criminal careers. Our aim was to assess factors associated with current criminal involvement and specialization in selling drugs, property crime and violence. METHOD: A sample of 1,039 frequent crack users was recruited in three major Dutch cities, combining respondent-driven sampling with random institutional sampling. Bivariate and logistic regression analyses were performed to find factors associated with current criminality. RESULTS: A total of 431 participants (41.5%) had engaged in crime in the past 30 days, mostly selling drugs (68.9%), followed by property crimes (34.4%) and a few cases of violent crime (9.7%). Younger age, homelessness, heavier patterns of use and a more prolific criminal justice history were associated with current criminality. Those receiving welfare benefits tended to be more likely to specialize only in selling drugs as opposed to (also) property crimes. CONCLUSION: Reducing drug use among criminally involved crack users and addressing their housing conditions could have a significant impact on reducing drug-related crime. Welfare benefits might act as protective factor against committing property crimes but not against the selling of drugs.

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

BACKGROUND AND AIMS: 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.

Risk factors of γ-hydroxybutyrate overdosing.

The aim of this study was to identify in recreational drug users the factors which increase the risk of overdosing (OD) with γ-hydroxybutyrate (GHB). A purposive sample of 45 experienced GHB users was interviewed, equally divided into three groups (never OD, occasional OD, and repeat OD). The repeat OD group scored highest on many risk factors regarding GHB use, the occasional OD group scored intermediate, and the never OD group scored lowest. Participants, whether or not they had overdosed on GHB, most often perceived GHB use (e.g. using more GHB than usual, using GHB doses too closely together) as the main reason for GHB OD, and many participants who had overdosed on GHB reported that they had taken more GHB than usual at their most recent occasion of GHB OD. No significant differences in co-use of GHB with other substances were found between the three groups. Our findings indicate that using GHB in the company of groups of friends probably reduces, but does not eliminate, the risk of OD.

Facilitators and barriers in treatment seeking for cannabis dependence.

BACKGROUND: Relatively few cannabis dependent people seek treatment and little is known about determinants of treatment seeking. METHODS: Treatment determinants were compared among 70 DSM-IV cannabis dependent patients and 241 non-treatment seeking DSM-IV cannabis dependent community subjects. In addition, perceived facilitators for treatment seeking were assessed in patients, whereas perceived barriers were assessed in 160/241 cannabis dependent community subjects not prepared to seek treatment (precluders), of whom 63/160 showed an objective treatment need, and 30/241 showed a subjective treatment need. RESULTS: Compared to non-treatment seekers, patients reported more cannabis use (176.9 versus 82.8 joints monthly), more symptoms of dependence (5.6 versus 4.5), higher perceived lack of social support (70.0% versus 41.1%), more pressure to seek treatment (58.6% versus 21.6%), a more positive attitude to treatment, and more previous treatments. In addition, patients reported more mental health problems (internalising disorders 57.1% versus 24.5%; externalising disorders 52.9% versus 35.3%) and more functional impairments (8.4 versus 4.8 monthly days out of role). Cannabis dependent 'precluders' reported desire for self-reliance (50.0%), preference for informal help (22.5%), and absent treatment need (16.9%) as their main reasons not to seek treatment, whereas cannabis dependent community subjects with a subjective treatment need mainly expressed desire for self-reliance (36.7%), treatment ineffectiveness (16.7%), and avoiding stigma (13.3%). CONCLUSIONS: Functional impairment, mental health problems and social pressure are important reasons to seek treatment in people with cannabis dependence. Treatment participation might improve if desire for self-reliance and the preference for informal help are considered, and perceived ineffectiveness of treatment and stigmatisation are publicly addressed.

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, 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).

Predicting the transition from frequent cannabis use to cannabis dependence: a three-year prospective study.

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.

Reliability and validity of the Severity of Dependence Scale for detecting cannabis dependence in frequent cannabis users.

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. Internal consistency and criterion validity of the SDS were investigated within an enriched community based sample of 577 Dutch frequent (≥ three days per week in the past 12 months) cannabis users between 18-30 years. Criterion validity was tested against the Composite International Diagnostic Interview (CIDI) 3.0 DSM-IV diagnosis cannabis dependence, and psychometric properties were assessed separately for males and females and for ethnic subgroups. Principal component analysis showed that all items of the scale loaded on a single factor and reliability of the SDS total score was good (Cronbach's α = 0.70). However, criterion validity against the CIDI diagnosis cannabis dependence was low: area under curve (AUC) was 0.68 (95% confidence interval: 0.64-0.73) and at the optimal differentiating cut-off (SDS ≥ 4), sensitivity was 61.3% and specificity 63.5%. Results were similar for subgroups on gender and ethnicity. While internal consistency of the SDS is good, its use as a screener to differentiate between dependence and non-dependence within populations of young adult frequent cannabis users is not recommended.

Validation of self-reported cannabis dose and potency: an ecological study.

AIMS: To assess the reliability and validity of self-reported cannabis dose and potency measures. DESIGN: Cross-sectional study comparing self-reports with objective measures of amount of cannabis and delta-9-tetrahydrocannabinol (THC) concentration. SETTING: Ecological study with assessments at participants' homes or in a coffee shop. PARTICIPANTS: Young adult frequent cannabis users (n = 106) from the Dutch Cannabis Dependence (CanDep) study. MEASUREMENTS: 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. FINDINGS: 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. CONCLUSIONS: 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.