Volkow ND, Baler RD, Compton WM, Weiss SR. Adverse health effects of marijuana use. N Engl J Med 2014;370(23):2219-27 Abstract

Why was this study conducted?

Patients who are considering medical cannabis may ask their physicians about potential adverse effects so the authors have tried to summarize the available evidence on psychological, cognitive and health risks. As other reviews have shown, the risks are greater for those who begin long-term or heavy recreational cannabis use early in adolescence.

What does this study add?

The authors rated their level of confidence in the evidence for the adverse effects of heavy or long-term cannabis use, particularly if it starts in adolescence. Their confidence in the evidence was high for addiction, diminished lifetime achievement, motor vehicle accidents, and bronchitis; medium for abnormal brain development, progression to other drugs, schizophrenia, and depression or anxiety; and low for lung cancer. They did not rate the level of the evidence on the short-term effects of cannabis use: impaired short-term memory and motor coordination, altered judgment and, in high doses, paranoia or psychosis. The authors also found some research suggesting that during intoxication cannabis may be associated with vascular changes that increase the risk of heart attack and stroke, but the mechanism underlying these findings are poorly understood and they said more research is needed. Some of the concerns raised in this review are reflected in the advice of the Canadian College of Physicians and Surgeons about who are not appropriate patients for medical cannabis.

Is there anything else I should know?

The literature reviewed by the authors included observational studies of recreational cannabis users and experiments with animals but there was little information from studies of medical cannabis. The authors note that multiple, often hidden, confounding factors may make it difficult to establish direct causal links between cannabis use and the observed effects.

Author Details

The latest scientific evidence on this topic was reviewed by the Centre's leadership team. This research summary is written by Corinne Hodgson, DHealth, assessed for accuracy by Co-Director Dr. James MacKillop, PhD, an expert in addictions and mental health research. There are no conflicts of interest. Questions regarding this piece should be directed to Dr. James MacKillop (jmackill@mcmaster.ca).