Adverse Effects of Cannabis: What We've Learned in 20 Years

the MPR take:

Research on cannabis use conducted over the past 20 years have provided insight on adverse effects such as an increased risk of psychosis and cardiovascular events, but many of the findings do not account for confounders or are unable to establish a causal relationship. Published in the journal Addiction, a review of studies on cannabis use from 1993-2013 highlights the following points:

  • Cannabis users who drive under the influence have a 2-3 times greater risk of automobile accidents vs. those who do not use cannabis; those who drive while under the influence of cannabis and have elevated blood alcohol levels have a substantially increased risk.
  • The risk of developing psychosis is double among regular cannabis users. However, studies on psychosis incidence and cannabis use have led to mixed results and a causal relationship cannot be established. Those with a pre-existing suicide risk may have a greater risk if they are also a heavy cannabis user.
  • Respiratory function has been shown to decline in daily cannabis smokers but increase in those smoking the equivalent of 3-5 marijuana cigarettes per month.
  • A "reverse gateway" effect has been seen in the past 20 years, where cannabis use is now often initiated prior to cigarette smoking.
  • While regular cannabis users have been shown to have poorer educational attainments, studies on twins have been unable to confirm a causal relationship.
  • More cannabis users are seeking treatment and evidence for cannabis withdrawal syndrome is increasing. Symptoms include anxiety, insomnia, appetite disturbance, and depression. Cognitive behavioral therapy (CBT) may substantially reduce the frequency of cannabis use.
  • Evidence suggests that those who begin smoking in adolescence and continue with near-daily use in adulthood experience a decline in IQ, but larger neuroimaging studies with stricter study controls are needed to assess brain damage and cannabis use.
  • There has been consistent evidence that cannabis use is associated with adverse cardiovascular effects in middle-aged or older users. Younger adults that are cannabis users with undiagnosed cardiovascular disease may also be at an increased risk.
  • Cannabis smoke has been established to contain carcinogens, but many studies on cannabis use and cancer risk have not accounted for tobacco use as a confounder.

Study: Cannabis Use a Possible CVD Risk Factor
Adverse Effects of Cannabis: What We've Learned in 20 Years

Aims: To examine changes in the evidence on the adverse health effects of cannabis since 1993. Methods: A comparison of the evidence in 1993 with the evidence and interpretation of the same health outcomes in 2013.