Comprehensive review found recreational cannabis harms were generally minor but could be serious in vulnerable individuals
A systematic methodology review concluded that adverse effects of low-level recreational cannabis use were generally minor, and that overall cannabis harms appeared less serious than those of alcohol.
Quick Facts
What This Study Found
This comprehensive review examined multiple categories of cannabis adverse effects using systematic methodology.
Acute effects included mental slowness, impaired reaction times, and occasionally heightened anxiety. Serious psychological episodes occurred with high intoxication levels. Cannabis doubled the risk of fatal road accidents, though alcohol played an even greater role.
Regarding psychosis, several longitudinal cohort studies showed statistical associations between cannabis use and psychotic illness, but methodological problems (including unreliable self-reporting) prevented establishing a causal relationship. Notably, Australia's marked increase in cannabis use was not accompanied by increased schizophrenia incidence.
Cannabis dependence was described as usually psychological, with withdrawal symptoms (irritability, anxiety, sleep difficulties) appearing within 48 hours and resolving within 2-12 weeks. The review found inconclusive evidence on memory effects, depression, and suicide.
The overall conclusion was that adverse effects of low-level recreational use were generally minor, with cannabis harms appearing less serious than alcohol's.
Key Numbers
Cannabis doubled fatal road accident risk. Withdrawal symptoms within 48 hours, resolving in 2-12 weeks. Australia's cannabis increase did not produce matching schizophrenia increase.
How They Did This
Review conducted using Prescrire's standard systematic methodology, examining evidence across multiple outcome domains including neuropsychological effects, psychosis, dependence, driving, cardiovascular effects, cancer, and hepatitis C.
Why This Research Matters
The comparative framing (cannabis versus alcohol harms) and the conclusion about limited evidence for causation in the psychosis relationship were significant for public health policy discussions.
The Bigger Picture
This review represented a balanced assessment by a medication evaluation publication known for methodological rigor, positioning cannabis harms relative to alcohol in the broader landscape of substance-related health effects.
What This Study Doesn't Tell Us
The review acknowledged difficulty evaluating long-term effects due to confounding by tobacco, alcohol, and lifestyle factors. Evidence for several outcomes (cancer, hepatitis C, memory) was described as inconclusive.
Questions This Raises
- ?Will longer-term studies eventually clarify the causal question regarding cannabis and psychosis?
- ?How should cannabis-specific risks be weighed against alcohol risks in policy decisions?
Trust & Context
- Key Stat:
- Cannabis harms appeared less serious than those of alcohol overall
- Evidence Grade:
- Review using systematic methodology across multiple outcome domains, though many individual evidence areas were described as inconclusive.
- Study Age:
- Published in 2011. Both the evidence base and policy landscape have evolved considerably since.
- Original Title:
- Adverse effects of cannabis.
- Published In:
- Prescrire international, 20(112), 18-23 (2011)
- Authors:
- Database ID:
- RTHC-00465
Evidence Hierarchy
Summarizes existing research on a topic.
What do these levels mean? →Frequently Asked Questions
Is cannabis safer than alcohol?
This review concluded that overall adverse effects of cannabis appeared less serious than alcohol in terms of neuropsychological effects, physical harm, accidents, and violence. However, individual risks vary, and cannabis may pose specific dangers for vulnerable populations.
Does cannabis cause schizophrenia?
The review found statistical associations between cannabis and psychosis in several cohort studies, but concluded that a causal relationship could not be established due to methodological limitations. Australia's experience (more cannabis use without more schizophrenia) further complicated the picture.
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Cite This Study
https://rethinkthc.com/research/RTHC-00465APA
. (2011). Adverse effects of cannabis.. Prescrire international, 20(112), 18-23.
MLA
. "Adverse effects of cannabis.." Prescrire international, 2011.
RethinkTHC
RethinkTHC Research Database. "Adverse effects of cannabis." RTHC-00465. Retrieved from https://rethinkthc.com/research/unknown-2011-adverse-effects-of-cannabis
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.