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Date: February 20, 1996 For Release: 4:00 p.m. EST Contact: NIH/Sheryl Massaro (301) 443-6245
A new study shows critical skills related to attention, memory, and learning are impaired among heavy users of marijuana, even after discontinuing its use for at least 24 hours. "This casts serious doubt on the common belief among many marijuana users that they are fine once the marijuana high wears off," says Alan I. Leshner, Ph.D., Director of the National Institute on Drug Abuse (NIDA), National Institutes of Health, which funded the study. "All along, we've been telling young people not to smoke marijuana, especially if they want to do well in school. Now we know that, for students who smoke marijuana heavily, the ability to learn is affected not just while they are high, but for at least a day after."
The study was conducted by Harrison G. Pope, Jr., M.D. and Deborah Yurgelun-Todd, Ph.D. at the McLean Hospital in Belmont, Massachusetts from 1991 to 1994 and is published in the February 21, 1996 issue of the Journal of the American Medical Association.
In the single-blind study of college undergraduate men and women, the researchers compared 65 "heavy users," who had smoked marijuana a median of 29 of the past 30 days, and 64 "light users," who had smoked a median of 1 of the past 30 days. Students were recruited based on self-reports of marijuana use during the past 2 years, as well as use of other illicit drugs and alcohol and tobacco. The study controlled for such traits as baseline intelligence levels, possible psychiatric disorders, family background, and use of other substances prior to the study period.
After a closely-monitored 19- to 24-hour period of abstinence from marijuana and other illicit drugs and alcohol, the undergraduates were given several standard tests measuring aspects of attention, memory, and learning, such as general intellectual functioning, abstraction ability, attention span, verbal fluency, and the ability to learn and recall new verbal and visual-spatial information.
The heavy users made more errors and had more difficulty in sustaining attention, shifting attention to meet the demands of changes in the environment, and in registering, processing, and using information (referred to as "executive cortical function") in comparison to the light users. Men in the heavy use group showed somewhat greater impairment than women in the same group.
"The heavy users," says Dr. Pope, "could not pay attention to the material well enough to register the information in the first place so that it could be recalled and repeated later."
"We know from NIDA-funded research," says Dr. Leshner, "that daily marijuana use among young people has increased in recent years. Young people are putting themselves at high risk of failure due to their marijuana use."
The researchers suggest the finding of greater impairment among the heavy users is likely due to an alteration of brain activity produced by marijuana, a residue of the drug in the brain, or an actual drug withdrawal syndrome from marijuana.
"We still do not know," says Dr. Pope, "whether certain effects of marijuana use are only short term or may also have much longer-term implications, even after all traces of marijuana have left the system." Continued research is planned to determine the neuropsychological function of long-term heavy marijuana users for up to 28 days after use is discontinued.
The National Institute on Drug Abuse, part of the National Institutes of Health, is the primary Federal agency responsible for basic, clinical, and applied research designed to improve and develop new strategies to deal with the health problems and issues associated with drug abuse and addiction.