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