title: Current and former marijuana use: preliminary findings of a longitudinal study of effects on IQ in young adults.
abstract: BACKGROUND
  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.
  
  
  METHODS
  We determined marijuana use for seventy 17- to 20-year-olds through self-reporting 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 non-users (who never smoked more than once per week and no smoking in the past two weeks).
  
  
  RESULTS
  Current marijuana use was significantly correlated (p < 0.05) in a dose-related 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 non-users (2.6).
  
  
  CONCLUSIONS
  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 long-term 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.
authors.affiliation: Department of Psychology, Carleton University, Ottawa, Ont. peter_fried@carleton.ca
authors.forename: Peter
authors.initials: P
authors.lastname: Fried
authors.forename: Barbara
authors.initials: B
authors.lastname: Watkinson
authors.forename: Deborah
authors.initials: D
authors.lastname: James
authors.forename: Robert
authors.initials: R
authors.lastname: Gray
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000704:Analysis of Variance; D002188:Cannabis / Q000009:adverse effects*; D003071:Cognition / Q000187:drug effects; D003072:Cognition Disorders / Q000139:chemically induced* / Q000453:epidemiology; D005260:Female; D006801:Humans; D007361:Intelligence Tests; D008137:Longitudinal Studies; D008297:Male; D009864:Ontario / Q000453:epidemiology; D011237:Predictive Value of Tests; D011336:Probability; D012016:Reference Values; D018570:Risk Assessment; D012307:Risk Factors; D012494:Sampling Studies; D019966:Substance-Related Disorders / Q000150:complications* / Q000453:epidemiology; D013997:Time Factors