Effects of marijuana on young adults

Peter Fried and colleagues1 found that heavy current use of marijuana decreases IQ scores by approximately 5 units in young adults over their adolescent years, compared to light or non-users. Study participants who were heavy users as recently as 3 months prior to the study had no decrease in their IQ scores, suggesting that the effects on IQ are reversible once use is discontinued. The authors suggest that a decrease of 5 units is important because it would increase the percentage of people with an IQ below the cut-off points for intervention and special education — below 77.5 from 6.7% to 11.0%, and below 70 from 2.3% to 5.5%. A similar decline would occur among those with IQs above 122.5 and 130.

It would be interesting to see the results of such an analysis using the authors' data. Was there an increase in the proportion of people below 77.5? Because of its effects on motivation, it is possible that marijuana use affected the IQ scores of participants who scored high initially but not the IQ score of participants who scored low initially. Another way to address this question would be to include the initial IQ score as an independent variable. This might also explain the lack of an effect on participants who were smoking an average of 37 joints/week but had stopped at least 3 months previously.

Ian Shrier President, Canadian Academy of Sport Medicine Centre for Clinical Epidemiology and Community Studies The Sir Mortimer B. Davis–Jewish General Hospital Montreal, Que.