title: Early-onset cannabis use and cognitive deficits: what is the nature of the association?
abstract: BACKGROUND
  Individuals who initiate cannabis use at an early age, when the brain is still developing, might be more vulnerable to lasting neuropsychological deficits than individuals who begin use later in life.
  
  
  METHODS
  We analyzed neuropsychological test results from 122 long-term heavy cannabis users and 87 comparison subjects with minimal cannabis exposure, all of whom had undergone a 28-day period of abstinence from cannabis, monitored by daily or every-other-day observed urine samples. We compared early-onset cannabis users with late-onset users and with controls, using linear regression controlling for age, sex, ethnicity, and attributes of family of origin.
  
  
  RESULTS
  The 69 early-onset users (who began smoking before age 17) differed significantly from both the 53 late-onset users (who began smoking at age 17 or later) and from the 87 controls on several measures, most notably verbal IQ (VIQ). Few differences were found between late-onset users and controls on the test battery. However, when we adjusted for VIQ, virtually all differences between early-onset users and controls on test measures ceased to be significant.
  
  
  CONCLUSIONS
  Early-onset cannabis users exhibit poorer cognitive performance than late-onset users or control subjects, especially in VIQ, but the cause of this difference cannot be determined from our data. The difference may reflect (1). innate differences between groups in cognitive ability, antedating first cannabis use; (2). an actual neurotoxic effect of cannabis on the developing brain; or (3). poorer learning of conventional cognitive skills by young cannabis users who have eschewed academics and diverged from the mainstream culture.
authors.affiliation: Department of Psychiatry, Harvard Medical School, McLean Hospital, 115 Mill Street, Belmont, MA 02478, USA. pope@mclean.harvard.edu
authors.forename: Harrison G
authors.initials: HG
authors.lastname: Pope
authors.forename: Amanda J
authors.initials: AJ
authors.lastname: Gruber
authors.forename: James I
authors.initials: JI
authors.lastname: Hudson
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Cohane
authors.forename: Marilyn A
authors.initials: MA
authors.lastname: Huestis
authors.forename: Deborah
authors.initials: D
authors.lastname: Yurgelun-Todd
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001921:Brain / Q000187:drug effects; D002186:Cannabinoids / Q000008:administration & dosage / Q000009:adverse effects*; D003072:Cognition Disorders / Q000139:chemically induced* / Q000175:diagnosis; D005260:Female; D006801:Humans; D008297:Male; D002189:Marijuana Abuse / Q000150:complications* / Q000534:rehabilitation; D008875:Middle Aged; D009483:Neuropsychological Tests; D012307:Risk Factors
chemical_list: D002186:Cannabinoids