The University of California at San Diego (UCSD) study looked at the mental function of regular marijuana smokers when they were not high. They used 15 previously published studies on the impact of long-term marijuana use to reach their conclusions.

"The findings were kind of a surprise. One might have expected to see more impairment of higher mental function," said Dr. Igor Grant, a UCSD professor of psychiatry and lead author of the study.

While marijuana affects perception, the researchers determined that it has only a slight long-term damaging effect on learning and memory. The research showed no long-term effect on reaction time, attention, language, reasoning ability, and perceptual and motor skills.

The study's results could prove significant in the medical-marijuana debate. "If we barely find this tiny effect in long-term heavy users of cannabis, then we are unlikely to see deleterious side effects in individuals who receive cannabis for a short time in a medical setting," Grant said.

Anecdotal evidence suggests marijuana can alleviate pain or nausea in certain patients, though no controlled studies to date have demonstrated those effects. "If it turned out that new studies find that cannabis is helpful in treating some medical conditions, this enables us to see a marginal level of safety," Grant noted.

The reseachers also observed that heavy cannabis users often abuse other drugs and might incur long-term neurological damage from substances like alcohol or amphetamines.

Limitations in the analysis included the number of subjects in some of the studies analyzed, and inadequate information about factors like additional drug use in others.

The study's findings are published in the July 2003 issue of the Journal of the International Neuropsychological Society.

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