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.
Read this if you want a balanced overview of what the evidence says about cannabis health risks.
Cannabis harms appeared less serious than those of alcohol overall
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
Review using systematic methodology across multiple outcome domains, though many individual evidence areas were described as inconclusive.
When this study was published
Published in 2011. Both the evidence base and policy landscape have evolved considerably since.
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.
Questions still open
- 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?
Common questions
Is cannabis safer than alcohol?
Does cannabis cause schizophrenia?
Read the original research
Adverse effects of cannabis.
Prescrire international, 20(112), 18-23
Citation
. (2011). Adverse effects of cannabis.. Prescrire international, 20(112), 18-23.
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