Memory, IQ, math and verbal school performance, and occupational performance have all been shown to suffer due to marijuana use.  This is in addition to sizeable increases in depression and schizophrenia covered on other pages.

IQ Decreased Only in Current Heavy Users: IQ scores at age 9-12 were compared to IQ at age 17-20. Users of over 5 joints per week had a 4.1 IQ point decrease vs. an increase of 3 IQ points in the rest with no significant difference between light users, former users, and non-users. : CMAJ 2002 Apr 2;166(7):887-91. Canada.

Long-Term Use Hurts Memory: Long-term cannabis users (near daily for 24 years) performed significantly less well than shorter-term users and controls on tests of memory and attention. On the Rey Auditory Verbal Learning Test, long-term users recalled significantly fewer words than either shorter-term users (P =.001) or controls (P =.005); there was no difference between shorter-term users and controls. Long-term users showed impaired learning (P =.007), retention (P =.003), and retrieval (P =.002) compared with controls. Both user groups performed poorly on a time estimation task (P<.001 vs. controls). Performance measures often correlated significantly with the duration of cannabis use, being worse with increasing years of use, but were unrelated to withdrawal symptoms and persisted after controlling for recent cannabis use and other drug use. CONCLUSIONS: These results confirm that long-term heavy cannabis users show impairments in memory and attention that endure beyond the period of intoxication and worsen with increasing years of regular cannabis use. patients were being admitted to drug treatment. Seattle, JAMA 2002 Mar 6;287(9):1123-31

Memory Damage from Ecstacy and Marijuana: According to a survey study of 763 young adults worldwide, ecstacy damage long-term memory and marijuana short term memory and regular users of both had both types of memory damage. Ecstasy users were 23% more likely to have problems remembering things and cannabis users 20% more likely when compared to non-users. Also, people who regularly take ecstasy or smoke cannabis were more likely to have made mistakes when completing the questionnaires. Jacqui Rodgers, University of Newcastle, J Psychopharmacology 1/15/04

Long-Term Use Disrupts Memory: Long-term cannabis use (34 years) was associated with disruption of short-term memory, working memory, and attentional skills in older long-term cannabis users. Study of Costa Rica men matched. Short-term users (8 years) didn’t show difference from controls. U Tx. Arch Gen Psychiatry 1996 Nov;53(11):1051-7

Heavy Use (7/week) in Young Adults Associated with Poor Performance in Math, Verbal, Memory: Adult marijuana users and non-users (N = 144 and N = 72, respectively) matched on intellectual functioning before the onset of drug use, i.e., on scores from standardized tests administered during the fourth grade of grammar school (Iowa Tests of Basic Skills). Subjects were given the twelfth grade versions of these tests (Iowa Tests of Educational Development) and other, computerized cognitive tests in successive test sessions. "Heavy" marijuana use (defined by use seven or more times weekly) was associated with deficits in mathematical skills and verbal expression in the Iowa Tests of Educational Development and selective impairments in memory retrieval processes in Buschke's Test. The retrieval impairments were restricted to words that were easy to visualize. Impairments depended on the frequency of chronic marijuana use, i.e., "light" and "intermediate" marijuana use (defined by use one to four and five to six times weekly, respectively) were not associated with deficits. Psychopharmacology (Berl) 1993;110(1-2):219-28

Teens Memory Impairment: Study concluded that cannabis-dependent adolescents have selective short-term memory deficits that continue for at least 6 weeks after the last use of marijuana. Am J Dis Child 1989 Oct;143(10):1214-9

Long Term Use Hurts: 26 heavy cannabis users (mean duration of cannabis use 6.76 years; average daily intake 150 mg THC). Compared with a control group, the cannabis users were found to react slowly in perceptual-motor tasks, but not to differ in intelligence or memory tests. The users suffered disability in personal, social and vocational areas and indicated higher psychoticism and neuroticism scores. India. Drug Alcohol Depend 1988 May;21(2):147-52

School Dropout 130% Higher in Marijuana Users: A longitudinal survey of 1392 adolescents aged 16-18 years. The results suggest that marijuana initiation is positively related to dropping out of high school. Although the magnitude and significance of this relationship varies with age of dropout and with other substances used, it is concluded that the effect of marijuana initiation on the probability of subsequent high school dropout is relatively stable, with marijuana users' odds of dropping out being about 2.3 times that of non-users. Health Econ 2000 Jan;9(1):9-18

Marijuana, Alcohol Both Linked to Poorer Occupational Outcome: In a longitudinal study of 197 Afro-American youth found that the greater and earlier the marijuana use and the greater and earlier the alcohol use, the poorer occupational performance. Am J Drug Alcohol Abuse 1996 Feb;22(1):57-73

Life-Long Impairment in Rat Offspring: The offspring of pregnant rats given a low dose of cannabinoid were found to perform poorly in learning tests throughout their lives, compared to rats that were not exposed. La Sapienza Univ, Rome, Vincenzo Cuomo, Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0537849100)