Effects of marijuana on young adults

I have some questions regarding the report by Peter Fried and colleagues on the effect of marijuana use on IQ.1

The article stated that no significant difference was found in IQ difference scores between former users and non-users (3.5 v. 2.6). This may be because the young adults that fit the definition of former user (smoked marijuana at least once a week in the past and not smoked for the last 3 months) and non-user (never smoked marijuana at least once per week) may actually be quite similar in their marijuana use. For instance, a former user may have smoked marijuana only once a week for a very brief time in the past, while a non-user may have smoked marijuana once every 2 weeks but for a very long time. Can the authors provide more details of marijuana use among the individuals in the former user and non-user categories?

In addition, no significant difference in IQ difference scores was reported between former heavy users (at least 5 joints per week) and non-users. Because the sample size of former users (n = 9) was already small, and the number of former heavy users would be even lower than this, the lack of significance may be due simply to a lack of statistical power. Can the authors provide more details on the number of former heavy users and their IQ difference scores?

Paul J. Yong MD/PhD and Experimental Medicine Programs University of British Columbia Vancouver, BC