Neuropsychological consequences of regular marijuana use: a twin study.
BACKGROUND
Results of previous research examining long-term residual effects of marijuana use on cognition are conflicting. A major methodological limitation of prior studies is the inability to determine whether differences between users and non-users are due to differences in genetic vulnerability preceding drug use or due to the effects of the drug.


METHODS
Fifty-four monozygotic male twin pairs, discordant for regular marijuana use in which neither twin used any other illicit drug regularly, were recruited from the Vietnam Era Twin Registry. A minimum of 1 year had passed since the marijuana-using twins had last used the drug, and a mean of almost 20 years had passed since the last time marijuana had been used regularly. Twins were administered a comprehensive neuropsychological test battery to assess general intelligence, executive functioning, attention, memory and motor skills. Differences in performance between marijuana-using twins and their non-using co-twins were compared using a multivariate analysis of specific cognitive domains and univariate analyses of individual test scores. Dose response relationships were explored within the marijuana-using group.


RESULTS
Marijuana-using twins significantly differed from their non-using co-twins on the general intelligence domain; however, within that domain only the performance of the block design subtest of the Wechsler Adult Intelligence Scale--Revised reached a level of statistical significance.


CONCLUSIONS
Out of the numerous measures that were administered, only one significant difference was noted between marijuana-using twins and their non-using co-twins on cognitive functioning. The results indicate an absence of marked long-term residual effects of marijuana use on cognitive abilities.
Lyons
M J
MJ
Psychology Department, Boston University, Boston, MA 02215, USA. mlyons@bu.edu
Bar
J L
JL
Panizzon
M S
MS
Toomey
R
R
Eisen
S
S
Xian
H
H
Tsuang
M T
MT
D000328:Adult; D000704:Analysis of Variance; D004200:Diseases in Twins / Q000235:genetics* / Q000523:psychology; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007360:Intelligence / Q000187:drug effects; D008297:Male; D002189:Marijuana Abuse / Q000235:genetics* / Q000523:psychology; D008385:Marijuana Smoking / Q000009:adverse effects*; D008875:Middle Aged; D009483:Neuropsychological Tests / Q000706:statistics & numerical data*; D011594:Psychometrics / Q000706:statistics & numerical data; D019966:Substance-Related Disorders / Q000235:genetics* / Q000523:psychology; D014430:Twins, Monozygotic / Q000235:genetics / Q000523:psychology; D014888:Wechsler Scales / Q000706:statistics & numerical data