"It appears that cognitive impairment from marijuana use is temporary and related to the amount of marijuana that has been recently smoked rather than permanent and related to an entire lifetime consumption," said Dr. Harrison G. Pope, Jr. of McLean Hospital in Belmont, Mass., and lead author of the study.

Pope and fellow researchers studied three groups: those who smoked marijuana daily, occasionally, and not at all. All participants agreed to abstain from marijuana use during the 28-day study.

The researchers found that heavy marijuana users had significantly lower scores on word-recall lists at the beginning of the study compared with nonusers. But by day 28 of the tests, there were no significant differences between the groups in any of the tests.

"People who are regular heavy marijuana smokers will exhibit some impairment that lasts days, and possibly even a couple of weeks after they stop smoking -- that's the bad news," said Pope. "The good news is that if they abstain from marijuana for longer than four weeks, then the residual effects seem to disappear."

In a second study led by Dr. Alan J. Budney from the University of Vermont in Burlington, researchers found that the marijuana withdrawal symptoms experienced by habitual users are similar to those experienced by people who are trying to quit smoking tobacco.

"Comparing our results to studies of nicotine withdrawal, it looks like the magnitude of the severity of withdrawal is similar," Budney said. "So as people try to quit smoking marijuana, one can expect them to have problems with withdrawal."

The report is published in the October issue of the Archives of General Psychiatry.

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