In a Vancouver cohort of 357 heroin users, cannabis use was the strongest independent predictor of initiating heavy drinking, doubling the risk, while methadone treatment did not increase heavy drinking.
Read this if you are interested in how cannabis use relates to alcohol risk in people with opioid use disorders.
Cannabis use doubled the risk of heavy drinking initiation (HR = 2.06)
What the researchers found
There has been concern that people entering methadone maintenance therapy (MMT) for heroin addiction might substitute alcohol for heroin. This study followed 357 heroin users in Vancouver for up to 9 years to test this hypothesis.
Methadone treatment did not predict heavy drinking initiation. In fact, those enrolled in MMT at some point had lower heavy drinking rates than those who never enrolled (4.6 vs. 16.2 per 100 person-years).
The surprise finding was about cannabis. Cannabis use was independently associated with a 2.06-fold increased risk of initiating heavy drinking, making it the strongest substance-use predictor in the model. Younger age was the only other significant predictor.
Why it matters
This study provides two important findings: methadone treatment does not drive heroin users toward heavy drinking (a concern that could discourage treatment), and cannabis use is a surprisingly strong predictor of heavy drinking in this population. The cannabis finding suggests potential cross-substance vulnerability.
The numbers in context
357 heroin users followed. 208 (58%) enrolled in MMT during follow-up. 115 (32%) initiated heavy drinking. Incidence: 7.8 per 100 person-years overall. MMT enrolled: 4.6 vs. not enrolled: 16.2 per 100 person-years. Cannabis use: adjusted HR = 2.06 (95% CI 1.32-3.19). Younger age: adjusted HR = 0.74 per 10 years.
How the study worked
Prospective community-based cohort study of people who inject drugs in Vancouver, Canada, from December 2005 to May 2014. Extended Cox regression analysis examined the effect of methadone enrollment on heavy drinking onset, controlling for demographic and substance-use characteristics.
What this study cannot tell us
Observational study cannot establish causation. Cannabis use might be a marker for broader substance use vulnerability rather than a direct cause of heavy drinking. Self-reported substance use data. Vancouver's unique drug policy context may limit generalizability.
How to read the evidence
Prospective cohort design with extended Cox regression and multiple confounders controlled. Good methodology but observational limitations.
When this study was published
Published in 2016 using data from 2005-2014. The opioid crisis and treatment landscape have evolved significantly.
The bigger picture
The finding that cannabis use predicts heavy drinking initiation among heroin users adds to growing evidence of polysubstance vulnerability. It suggests that clinicians treating opioid use disorder should also monitor cannabis and alcohol use patterns.
Questions still open
- Why does cannabis use predict heavy drinking in heroin users? Is cannabis use a general marker of polysubstance vulnerability, or does it specifically increase alcohol risk? Should opioid treatment programs screen for and address cannabis use?
Common questions
Does methadone treatment lead to heavy drinking?
Why is cannabis use linked to heavy drinking in heroin users?
Read the original research
The Impact of Enrolment in Methadone Maintenance Therapy on Initiation of Heavy Drinking among People Who Use Heroin.
European addiction research, 22(4), 210-4
Citation
Klimas, Jan; Wood, Evan; Nguyen, Paul; Dong, Huiru; Milloy, Michael John; Kerr, Thomas; Hayashi, Kanna. (2016). The Impact of Enrolment in Methadone Maintenance Therapy on Initiation of Heavy Drinking among People Who Use Heroin.. European addiction research, 22(4), 210-4. https://doi.org/10.1159/000444513
Explore the wider topic
- Physical vs Psychological Cannabis Dependence: What Science Actually Shows
- The Gap Between Cannabis Perception and Science: Why What We Believe Outpaces What We Know
- Cannabis Use Disorder: Am I Addicted? Self-Assessment Guide
- Cross-Addiction After Quitting Weed: When One Habit Replaces Another
- Is Weed Addictive? What the Research Actually Shows
- Is Cannabis Addictive? What the DSM-5 and Brain Science Say
- Quitting Weed and Alcohol Together: What to Know About Dual Cessation
- Is Rehab Necessary for Weed? An Honest Assessment
- Signs You May Have Cannabis Use Disorder: An Honest Self-Assessment
- Weed Vape Pen Addiction: Why Vaping Makes It Harder to Quit