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Neuropsychological consequences of regular marijuana use: a twin study

  • M. J. LYONS (a1), J. L. BAR (a1), M. S. PANIZZON (a1), R. TOOMEY (a1), S. EISEN (a1), H. XIAN (a1) and M. T. TSUANG (a1)...


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.

Method. 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.


Corresponding author

Dr Michael J. Lyons, Psychology Department, Boston University, 64 Cummington Street, Boston, MA 02215, USA. (Email:


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