EFFECTS OF MARIJUANA ON YOUNG ADULTS

I was interested to read the article by Peter Fried and colleagues,
particularly since funding for such long-term, prospective research is
increasingly hard to find. The results of this study over the years have
been interesting and important. However, I am concerned on 2 counts.

First, it is universally accepted that intelligence is multifaceted.
Current research proposes as many as 9 different "intelligences," including
kinesthetic, interpersonal, emotional and motivational, as well as the more
traditionally understood verbal and spatial functions.

To use IQ difference scores as a dependent variable to measure the effects
of anything on the complexity of human cognitive functioning is at best
woefully simplistic. At worst, it perpetuates the dangerous and misguided
myth that human intelligence can be encapsulated in 1 score -- a belief
that is abused and misused by many professionals and lay people alike.

This "single score" notion not only misrepresents the whole field of modern
psychometrics in the area of intellectual assessment but also ignores how
marijuana and other drugs may affect some aspects of cognitive functioning
but not others. The investigation of any such differential effects would be
more challenging, but also far more useful and enlightening.

Second, assuming no error occurred on the table outlining the
characteristics of marijuana user groups, the authors chose not to discuss
a highly significant result. In the current heavy-user group, prenatal
exposure to marijuana was very significantly higher than in all other
groups (11.6 joints per week v. 1.4, 1.4 and 1.5 in the non-user, light
user and former-user groups, respectively; p ( 0.001). This suggests that
individuals who were exposed to marijuana in utero are more than 10 times
more likely to become heavy users in late adolescence and early adulthood.
Surely this is a major finding that should be highlighted and addressed?

Maggie Mamen

Psychologist Centrepointe Professional Services Nepean, Ont.

Reference

1. Fried P, Watkinson B, James D, Gray R. Current and former marijuana use:
preliminary findings of a longitudinal study of effects on IQ in young
adults. CMAJ 2002;166(7):887-91.

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

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

Reference

1. Fried P, Watkinson B, James D, Gray R. Current and former marijuana use:
preliminary findings of a longitudinal study of effects on IQ in young
adults. CMAJ 2002;166(7):887-91.

Peter Fried and colleagues 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.

Reference

1. Fried P, Watkinson B, James D, Gray R. Current and former marijuana use:
preliminary findings of a longitudinal study of effects on IQ in young
adults. CMAJ 2002;166(7):887-91.

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.

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.

Reference

1. Fried P, Watkinson B, James D, Gray R. Current and former marijuana use:
preliminary findings of a longitudinal study of effects on IQ in young
adults. CMAJ 2002;166(7):887-91.