Review: marijuana and health

June 14, 2014, 2:19 pm


Adverse Health Effects of Marijuana Use. Volkow ND et al. N Engl J Med 2014 Jun 5;370:2219-2227.

No abstract available

This review article by Dr. Nora Volkow and her colleagues at the National Institute on Drug Abuse is in most respects a rather fair and balanced look at the potential adverse effects of marijuana use. Among the potential negative effects discussed are:

  • impaired driving ability
  • impaired brain development during childhood and adolescence
  • impaired school performance with increased risk of dropping out
  • marijuana addiction and withdrawal

The authors reasonably state that other often-mentioned potential adverse effects, such as progression to using other illicit drugs and increased risk of mental illness, are difficult to evaluate because of multiple confounding factors.

I completely agree with two of the authors’ major points:

  1. Adolescents can suffer negative psychosocial consequences from using marijuana.
  2. Driving while under the influence of THC is dangerous.

However, I think that the impact and coherence of the paper are vitiated by several factors. As this is a broad survey of the medical literature, the authors cite many studies to support their premises and conclusions. Unfortunately, they do not seem to analyze these sources carefully, and — without the reader going back and critically evaluating each reference — it is impossible to determine whether or not the argument is based on good science. In particular, I looked at some of the articles cited to demonstrate that a large number of users become addicted to marijuana (according to dependence criteria in DSM-IV.) This argument seems based on some large-scale surveys with minimal methods and controls.

In addition to the medical review, this article has a political component. The authors make the following simple argument: since marijuana has adverse health effects on young people, and since decriminalizing or legalizing marijuana will increase its use, therefore such decriminalization would increase adverse health consequences. This reasoning is suspect because no state is proposing that marijuana use should legal for children or adolescents. It will be interesting to see what happens in Colorado and Washington in their experiment with legalization.  In addition, the authors do not consider the adverse social and health effects of criminalizing possession of small amounts of marijuana. Certainly having a criminal record or spending time in jail can have significant negative consequences. 


  1. Ian Mitchell Says:

    While the authors declare in their study that they have no conflict of interest, Dr. Volkow’s employment with NIDA lends a strong level of bias to this paper, especially with the selection of support articles. I have attempted to add some recent and forgotten articles to this review:

  2. Leon Says:


    Thank you for your comments. Earlier this week I looked at the Volkow article again in preparation for an upcoming podcast, and was struck even more by the bias evident in its discussion. As you note, this is especially apparent in Table 2, “Level of Confidence in the Evidence for Adverse Effects of Marijuana on Health and Well-Being.” Some of the conclusions in that table are not supported by evidence presented, and contradict other sections of the paper. You discuss these discrepancies very well in your post (click on the link above), and I’d recommend that any interested reader check out your excellent discussion there.