Marijuana: Dumb and Dumber
> 8/4/2014 10:37:30 AM

Current and former marijuana use preliminary findings of a longitudinal

study of effects on IQ in young†adults

Fried P, Watkinson B, James D, Gray R CMAJ


2002; 166(7); 88791


Assessing marijuana's impact on intelligence quotient (IQ) has been hampered by a lack of evaluation of subjects

before they begin to use this substance. Using data from a group of young people whom we have been following

since birth, we examined IQ scores before, during and after cessation of regular marijuana use to determine any

impact of the drug on this measure of cognitive function. We determined marijuana use for seventy 17to

20yearolds through selfreporting and urinalysis. IQ difference scores were calculated by subtracting each person's IQ

score at 9-12 years (before initiation of drug use) from his or her score at 17-20 years. We then compared the

difference in IQ scores of current heavy users (at least 5 joints per week), current light users (less than 5 joints

per week), former users (who had not smoked regularly for at least 3 months) and nonusers (who never smoked

more than once per week and no smoking in the past two weeks). Current marijuana use was significantly

correlated (p < 0.05) in a doserelated fashion with a decline in IQ over the ages studied. The comparison of the

IQ difference scores showed an average decrease of 4.1 points in current heavy users (p < 0.05) compared to

gains in IQ points for light current users (5.8), former users (3.5) and nonusers (2.6). Current marijuana use had a

negative effect on global IQ score only in subjects who smoked 5 or more joints per week. A negative effect was

not observed among subjects who had previously been heavy users but were no longer using the substance. We

conclude that marijuana does not have a longterm negative impact on global intelligence. Whether the absence of

a residual marijuana effect would also be evident in more specific cognitive domains such as memory and

attention remains to be ascertained.




Fried P, Watkinson B, James D, Gray R CMAJ


2002; 166(7); 88791

MEDLINE is the source for the citation and abstract for this record

Full Source Title

CMAJ : Canadian Medical Association journal = journal de l'Association medicale canadienne

NLM Citation ID

11949984.1 (PubMed ID)

Publication Type

Comparative Study

Journal Article

Research Support, U.S. Gov't, P.H.S.



Author Affiliation

Department of Psychology, Carleton University, Ottawa, Ont.


Fried P; Watkinson B; James D; Gray R

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