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NIDA. (2002, April 1). Cognitive Deficits Associated with Heavy Marijuana Use Appear to be Reversible. Retrieved from

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April 01, 2002
Margi Grady

Memory and learning problems caused by heavy marijuana smoking persist for at least a week after cessation of use of the drug, but they appear to resolve completely within a month, a NIDA-supported study shows. "Cognitive impairment from heavy marijuana use may linger for a week or longer, but it does not appear to be permanent," says Dr. Harrison Pope, Jr., who led the study at Harvard University's McLean Hospital in Belmont, Massachusetts. "Even if users smoke a lot, these tests suggest they can eventually recover," he says. As a followup to this research, Dr. Pope and his colleagues are using functional magnetic resonance imaging to determine whether the more sensitive assessment tool reveals cognitive effects that his pencil-and-paper tests could not detect.

Heavy Marijuana Users Report Lower Income and Less Education Than Control Subjects Information gathered on the Harvard study's participants revealed that both current and former long-term heavy users of marijuana had markedly lower income and less education than control subjects, even though the education and income of the two groups' parents were similar, says Dr. Harrison Pope, Jr., the study's lead investigator.

"Even though our study suggests that heavy users recover from cognitive deficits after 4 weeks of abstinence, it doesn't follow that marijuana is a benign substance," says Dr. Pope. He notes that the study revealed some startling demographic differences between long-term heavy users and a control group. Data collected on participants when they joined the study showed that those who had used marijuana heavily for many years had markedly lower income and education levels than the control group, regardless of whether they were still using the drug.

Dr. Pope and his colleagues examined cognitive function in 180 participants, including 63 current heavy marijuana users, 45 former heavy users, and 72 control subjects. Current heavy users had smoked marijuana a minimum of 5,000 times during their lives (the equivalent of at least once a day for 13 years) and were smoking at least 7 times a week at the beginning of the study. Former heavy users had also smoked at least 5,000 times in their lives but no more than 12 times during the previous 3 months. The control subjects had smoked at least once but no more than 50 times during their lives and no more than once in the previous year. Participants ranged in age from 30 to 55. All three groups were carefully screened for unrelated characteristics that might affect the study results.

All participants were required to remain abstinent from marijuana and other drugs for the course of the 28-day study and submit urine specimens that were used to confirm their abstinence. Their cognitive function was evaluated through standardized neuropsychological tests at study entry and on the 1st, 7th, and 28th days of the study. On study entry (day 0) and days 1 and 7, current heavy marijuana users scored significantly lower than control subjects on tests of verbal learning and memory. This finding confirms and extends the findings of previous studies by Dr. Pope's group and others that have shown impaired cognitive skills in heavy marijuana users for up to 3 days after use is stopped. By day 28, the difference between the scores of the control group and those of current heavy marijuana users disappeared.

Income and education data suggest consequences of heavy marijuana use.

Two findings allowed the researchers to conclude that the cognitive deficits were associated with recent heavy use rather than total lifetime use. First, former heavy users showed no significant difference from the control subjects on any of the tests on any of the testing days. Also, the researchers found a clear relationship between lower test scores and higher levels of marijuana residues in urine at the beginning of the study, but no relationship between test scores and total lifetime marijuana use.

"This study is particularly significant for treatment," says Dr. Steven Grant of NIDA's Division of Treatment Research and Development. "By stopping drug use, heavy marijuana users are able to regain their memory and learning functions. Still, we cannot say there are no consequences to heavy marijuana use: The income and education data suggest the opposite."


  • Fletcher, J.M., et al. Cognitive correlates of long-term cannabis use in Costa Rican men. Archives of General Psychiatry 53(11):1051-1057, 1996.
  • Pope, H.G., Jr., et al. Neuropsychological performance in long-term cannabis users. Archives of General Psychiatry 58(10):909-915, 2001.
  • Pope, H.G., Jr., and Yurgelun-Todd, D. The residual cognitive effects of heavy marijuana use in college students. Journal of the American Medical Association 275(7):521-527, 1996.
  • Solowij, N. Cannabis and Cognitive Functioning. Cambridge, UK: Cambridge University Press, 1998.