CHICAGO - Heavy marijuana use appears to impair cognitive function even after 24 hours of abstinence, new research suggests.
Researchers at Harvard University medical school in Boston recruited 65 college students who reported they were "heavy" users, having smoked marijuana a minimum of 22 out of 30 days prior to testing, and 64 who were "light" users, smoking a maximum nine days of the last 30.
Subjects were supervised overnight to allow any drug in their system to wear off, and on the second day underwent a battery of neuropsychological tests.
The heavy users of marijuana showed greater impairment than light users on attention/executive functions, particularly on tasks such as card sorting and learning word lists.
The differences remained after they controlled for factors such as different levels of cognitive functioning before beginning drug use, and the use of alcohol and other substances.
"Heavy marijuana use is associated with residual neuropsychological effects even after a day of supervised abstinence from the drug," the researchers concluded in their report, published in the Feb. 21 issue of the Journal of the American Medical Association.
"However, the question remains open as to whether this impairment is due to a residue of drug in the brain, a withdrawal effect from the drug or a frank neurotoxic effect of the drug."
The study's authors, Dr. Harrison Pope and Dr. Deborah Yurgelun-Todd (PhD), noted several questions are raised by their and other study findings.
"These questions include how long marijuana-associated neuropsychological impairment lasts, whether it's related to cumulative lifetime use, whether it interacts with normal aging and to what extent, if at all, it compromises the actual functioning of users in their daily lives.
"Only by further study of these issues can it be judged whether neuropsychological impairment associated with marijuana use should be considered a public health problem."
In an editorial accompanying the publication, Dr. Robert Block of the University of Iowa college of medicine pointed out that despite some 40 published studies on cannabis use, methodological difficulties in many of them make reliable evidence on the effects on cognition and brain function scarce.
"Some antidrug messages contain exaggerated claims concerning such adverse effects," he wrote, "but more responsible disseminators of information to the public have little ammunition."
Great strides in the area of neurofunctional imaging with positron emission tomography offer the best chance of defining more exactly the effects of heavy marijuana use on cognition, Dr. Block added.
"The recent resurgence in marijuana use among U.S. youth provides a compelling motivation conducting such studies."
Dr. Juan Negrete, professor of psychiatry at both McGill University and the University of Toronto, said the Harvard study adds to the weight of evidence showing cognitive impairment resulting from marijuana use.
Their results are interesting because the comparison of heavy users to occasional users rather than nonusers is suggestive of a dose-response relationship, Dr. Negrete said.
Overall, findings are consistent and coherent, including recent work from a group in Ottawa who've shown children of pot-smoking mothers are consistently more impaired in executive functions than children of nonsmoking mothers.
"These changes are not visible to the naked eye, but they are impairments in the highest brain functions, those most needed to function and adjust in a technological society," he said in an interview. "It is, in this sense, very worrisome. These youngsters -- and they are young in general -- are impairing themselves in an era when these demands are higher than ever. I think that's important."
Copyright © 1996 Maclean Hunter Publishing Limited
Reprinted with permission.
Internet Mental Health (www.mentalhealth.com) copyright © 1995-2011 by Phillip W. Long, M.D.