Study characteristics
| Investigator, year | Country | Study design | Sample size (% female) | Definition of cannabis use* | Outcome(s) of interest | Statistical analysis | Findings |
|---|---|---|---|---|---|---|---|
| Best et al.,30 1999 | UK | Cross-sectional | 200 (30) | Categorical: daily users, occasional users (not every day in previous month), nonusers | Illicit opioid use, polydrug use | ANOVA, post hoc Scheffe test, linear regression Factors adjusted for: use of various substances, appetite, overall health, depression, anxiety | Cannabis nonusers had more occasions of heroin use than occasional and daily users Cannabis nonusers consumed significantly more alcohol and crack cocaine than occasional and daily users |
| Epstein et al.,31 2003 | US | Secondary analysis (3 separate analyses), 12 mo | 408 (40.4) | Dichotomized cannabis use and cannabis abuse/dependence diagnosis | Illicit opioid use, treatment retention, polydrug use, criminal activity | Cox proportional hazards regression Factors adjusted for: not stated Confounders in the regression | No significant association between cannabis use and illicit opioid use No significant association between cannabis use and treatment retention Suggests an association between cocaine abstinence and cannabis use Cannabis use category not associated with any differences in criminal activity |
| Levine et al.,32 2015 | US | Retrospective cohort, 1 yr | 290 (40.3) | Dichotomized cannabis use | Illicit opioid use, treatment retention | Logistic regression Factors adjusted for: total years of use of various substances | Not significant, but statistics not reported Cannabis-negative urine associated with treatment retention in both men and women |
| Lions et al.,33 2014 | France | Secondary RCT analysis, 45 wk | 158 (15.2) | Dichotomized: daily users v. nondaily users | Illicit opioid use | Univariate logistic regression Factors adjusted for: not stated | Pretreatment daily cannabis: OR 1.46 (95% CI 0.61–3.77), NS In-treatment daily cannabis: OR 2.81 (95% CI 1.22–6.48) |
| Nava et al.,34 2007 | Italy | Prospective cohort, 12 mo | 121 (14) | Dichotomized: long-term users (> 6 mo) and currently smoking at least 7 times per week v. nonusers never exposed to marijuana smoking | Illicit opioid use, treatment retention | Hierarchical linear modelling Factors adjusted for: not stated | Significant association between cannabis use and illicit opioid use No significant association between cannabis use and treatment retention |
| Nirenberg et al.,35 1996 | US | Prospective cohort, 6 mo | 70 (1.4) | Dichotomized cannabis use and categorical (4 groups) | Illicit opioid use, polydrug use | ANOVA Factors adjusted for: not stated | No significant association between cannabis use and illicit opioid use No significant difference in use of cocaine or benzodiazepine between 4 cannabis groups |
| Proctor et al.,36 2016† | US | Retrospective cohort, 12 mo | 2410 (40.4) | Dichotomized cannabis use | Illicit opioid use | Logistic regression Factors adjusted for: age, gender, employment status, ethnicity, marital status, average daily methadone dosage | Intake cannabis values in relation to opioid use at 4 time points: 3 mo: OR 1.17 (95% CI 0.83–1.63); 6 mo: OR 0.59 (95% CI 0.32–1.10); 9 mo: OR 0.63 (95% CI 0.24–1.66); 12 mo: OR 0.23 (95% CI 0.05–1.16) |
| Saxon et al.,37 1996 | US | Prospective cohort, 18 mo | 353 (38.2) | Categorical: 7-point scale ranging from 0 (never) to 6 (≥ 4 times per day) | Illicit opioid use, treatment retention, polydrug use | Cox regression model Factors adjusted for: age, gender, previous methadone treatment, substance use | No significant association between cannabis use, and illicit opioid use or treatment retention Significant association between cannabis use and cocaine use |
| Scavone et al.,13 2013 | US | Retrospective cohort, 9 mo | 91 (36.6) | Dichotomized cannabis use | Illicit opioid use, treatment retention, polydrug use | ANCOVA, parallel ANCOVA Factors adjusted for: daily opioid expenditure | No significant relation between frequency of cannabis use in treatment and opiate use No significant association between cannabis use and treatment retention Correlation between rates of cannabis use and illicit benzodiazepine use |
| Somers et al.,38 2012 | Ireland | Retrospective cohort, 15 mo | 123 (NR) | Dichotomized cannabis use | Illicit opioid use | Logistic regression Factors adjusted for: variables that were significant from univariate analysis, did not explicitly state which ones | Baseline: OR 0.88 (95% CI 0.67–1.15); 3 mo: OR 0.79 (95% CI 0.58–1.10); 9 mo: OR 0.78 (95% CI 0.55–1.20); 15 mo: OR 1.45 (95% CI 0.82–2.50); Total: adjusted OR 0.32 (95% CI 0.06–1.66) |
| Wasserman et al.,20 1998 | US | Prospective cohort, 6 mo | 74 (40.5) | Dichotomized cannabis use | Illicit opioid use | Cox proportional hazards regression Factors adjusted for: abstinence goals, positive moods, pleasant events, negative moods, life events, perceived stress, opioid withdrawal symptoms | Significant association between cannabis use and illicit opioid use |
| Zielinski et al.,14 2017 | Canada | Cross-sectional | 777 (46.7) | Dichotomized cannabis use in previous 30 d | Illicit opioid use | Multivariable logistic regression analysis Factors adjusted for: age, gender, methadone dosage, treatment duration | No significant association between cannabis use and illicit opioid use |
| Joe et al.,39 1998 | US | Prospective cohort, 360 d | 981 (39) | Dichotomized: at least weekly marijuana use or not | Treatment retention | Hierarchical linear regression model Factors adjusted for: age, ethnicity, marital status, legal status, employment status, number of lifetime arrests | No significant association between cannabis use and treatment retention |
| Peles et al.,40 2006 | Israel | Prospective cohort, 11 yr | 492 (27.2) | Dichotomized cannabis use | Treatment retention | Fisher exact test, Cox regression analysis Factors adjusted for: age, children, methadone dosage, use of various substances | No significant association between cannabis use and treatment retention |
| Peles et al.,41 2008 | US, Israel | Prospective cohort, 12 mo | 794 (31.0) | Dichotomized cannabis use | Treatment retention | Kaplan–Meier survival analysis with log rank for cumulative retention, Cox regression Factors adjusted for: methadone dosage, age | No significant association between cannabis use and treatment retention |
| Schiff et al.,42 2007 | Israel | Retrospective cohort, 13 mo | 2683 (14.1) | Dichotomized cannabis use | Treatment retention | Logistic regression Factors adjusted for: age, gender | Significant relation between cannabis use and increased treatment retention |
| Weizman et al.,43 2004 | Israel | Prospective cohort, 12 mo | 283 (NR) | Dichotomized: cannabis abuse v. not | Treatment retention, polydrug use, risk behaviours for HIV infection | Cox regression survival analysis Factors adjusted for: heroin, cocaine and benzodiazepine abuse | No significant association between cannabis use and treatment retention Significant association between cannabis use and use of benzodiazepine, amphetamine and cocaine Cannabis use was not related to any risk behaviours (statistics not reported) |
| White et al., 201417 | US | Retrospective cohort, 15–17 mo | 604 (39.4) | Dichotomized cannabis use | Treatment retention | χ2 test, Fisher exact test Factors adjusted for: not stated | At baseline, cannabis use was significantly associated with treatment retention |
| Bleich et al.,44 1999 | Israel | Prospective cohort, 12 mo | 148 (29.8) | Positive result of urine test for cannabis during 12th mo of treatment‡ | Polydrug use | χ2 test Factors adjusted for: not stated | Benzodiazepine abusers were more likely to currently abuse cannabis than nonabusers of benzodiazepine |
| Peirce et al.,45 2009 | US | Secondary RCT analysis, 12 wk | 386 (44) | Cannabis use, defined as positive result of testing of urine/breath sample obtained at study intake | Polydrug use | Mixed-model regression Factors adjusted for: age, gender, ethnicity, employment status, criminal activity, additional demographic factors | Significant association between cannabis use and stimulant use |
| Saxon et al.,46 1993 | US | Cross-sectional | 98 (0) | Dichotomized cannabis use | Polydrug use | Mann–Whitney test Factors adjusted for: not stated | Significant association between cannabis use and other drug use |
| Strain et al.,47 1991 | US | Cross-sectional | 66 (45) | Dichotomized: those with v. without history of cannabis use diagnosis | Polydrug use | Z-test Factors adjusted for: not stated | No significant association between cannabis use and use of alcohol, sedatives and cocaine |
| Bell et al.,48 1997 | Australia | Prospective cohort, 12 mo | 304 (43.1) | Continuous: average daily use of cannabis in previous month | Criminal activity | Multiple linear regression Factors adjusted for: age, gender, employment, benzodiazepine use, cost of drugs | Cannabis use was significant predictor of criminal activity at 12 mo |
Note: ANCOVA = analysis of covariance, ANOVA = analysis of variance, CI = confidence interval, NR = not reported, NS = not significant, OR = odds ratio, RCT = randomized controlled trial.
↵* “Dichotomized cannabis use” means users versus nonusers or at least 1 positive urine screen result versus none, unless specified otherwise.
↵† This study had too many results to present in this table, so we included only intake cannabis values in relation to opioid use at all time points. See study for more results.
↵‡ An abuser of any substance of abuse was defined as having a positive urine test result for that substance during the 12th month of treatment.