Effects of marijuana on young adults

Maggie Mamen claims that using IQ ignores the possibility that marijuana may have an impact on multiple domains of cognitive functioning. Far from being ignored, this was raised in both the interpretation of the results and in the abstract of our article.1 We emphasized that the drug's impact on particular domains of intelligence (memory and attention were highlighted) must be ascertained and may differ from a broadly based measure of intelligence represented by IQ. This is ongoing in our research at the moment.

IQ was chosen for our report of preliminary findings for many reasons. Primarily, this work was intended to contrast our findings with previous research in which no premorbid cognitive values were available. The vast majority of these studies employed IQ as a concurrent outcome variable and thus our use of IQ was a matter of permitting an “oranges to oranges” comparison with the other studies. Furthermore, although intelligence is unquestionably multifaceted, the IQ measure remains a major, widely used barometer of intellectual performance. For example, in most school boards (including the 2 boards in the Ottawa , Ont. area) it is the cornerstone of initial assessment of children and, in examining the outcome of exposure to drugs (prenatal or concurrent), IQ values are ubiquitously cited.

Regarding the use of marijuana during pregnancy, our research group has published over 100 papers on prenatal marijuana exposure and its impact on a host of outcomes. This putative relationship will, in fact, be the subject of future reports. However, (in part due to space limitations imposed by CMAJ) for the purposes of the published article, prenatal exposure was found not to be associated with IQ change.

Paul Yong suggests a possible similarity in marijuana use between former users and non-users. What follows is a more detailed description of these 2 groups. Of the 37 non-users, 18 had never used marijuana and 19 had used marijuana but never at the level of at least once a week. Of the 19 non-users who had tried marijuana in the past, only 7 had used it in the past year. In contrast, all 9 of the former users had smoked at least once a week previously with an average use of 21.4 joints per week for an average of 2.2 years.

Regarding the power issue with the sample size of heavy former users, 5 former heavy users showed an average IQ difference score of 0.8, which did not differ significantly from the non-users. Definitely, this comparison suffers from lack of statistical power and was meant only as an initial foray to compare heavy former use with heavy current use.

Ian Shrier asks if there was an increase in the proportion of people below 77.5 in our sample. The lowest IQ measured in our relatively small sample was 84. The projection of our results onto standard cutpoints was an example of the potential impact on society at large.

Shrier's suggestion that heavy users with higher IQ scores initially might be affected differentially from those initially scoring lower is interesting. Although not originally addressed in our article, that analysis has now been done. The correlation between initial IQ and the IQ difference score is not significant (r = –0.34, p = 0.21). In addition, when the initial IQ is dichotomized at the median, no difference exists between the IQ difference scores (F = 0.21, p = 0.65). When former users were examined in the same fashion, the results were the same, and no effect on difference scores was related to initial IQ.

Peter A. Fried Barb Watkinson Robert Gray Department of Psychology Carleton University Ottawa, Ont.