Investigators found that, among pot smokers seeking treatment for marijuana dependence, long-time users performed more poorly on tests of memory and attention than shorter-term users and non-users.

The findings show that over time, marijuana smoking can cause intellectual impairments that "endure beyond the period of intoxication" and worsen the longer a person uses the drug.

But another researcher not involved in the study pointed to shortcomings in the work that he says make it tough to draw that conclusion.

An accompanying editorial notes that marijuana users who seek drug treatment do not necessarily reflect users in general, since these individuals may have other health issues such as anxiety or depression.

Researchers studied 102 pot-smoking Americans and 33 nonusers Marijuana users typically smoked every day, with longtime users doing so for an average of 24 years. Shorter-term users had smoked for about 10 years, on average. The vast majority said that they currently were not using other drugs, or did so only occasionally.

Results of the mental functioning tests -- taken after at least 12 hours of abstinence -- showed that longtime users performed less well than shorter-term users and nonusers

Long-term users had problems with learning, storage of learned information and retrieval of information from memory. This does not mean the drug caused brain damage in these cases, as the impairments seen in longtime users were relatively subtle.

However, the deficits could affect daily functioning -- hindering, for example, a person's ability to study or remember an item he or she just read.

But although such deficits, if prolonged or irreversible, would be of "grave concern, other studies have found no such impairments in longtime marijuana users, according to a Harvard researcher.

Part of the difficulty in sizing up the impact of marijuana is weeding the drug's effects out from the "background noise" of other factors, like psychiatric problems and abuse of other drugs. In this study, nearly half of the longtime marijuana users had in the past regularly used or abused alcohol or other drugs.

But there are also plausible biological reasons for why sustained marijuana use could affect things like memory. The brain receptors the drug acts on exist in large numbers in regions involved in memory. Over the years marijuana exposure might change the way these receptors and other brain chemicals operate.

It seems almost certain that marijuana produces short-lasting mental deficits, but whether they endure or worsen over time is still unclear.

Also unknown is whether any impairments are reversed after a person stops smoking pot.

JAMA March 6, 2002;287:1123-1131, 1172-1174