SAN FRANCISCO (San Francisco Chronicle) - A three-city study of heavy marijuana users has found that longterm pot smoking impairs brain function, scientists report.

In an elaborate study of more than 150 men and women being treated for dependence on weed, the researchers concluded that even many hours after the subjects' last joint, their memory proved defective, and so was their ability to concentrate, to solve problems involving numbers and words, and to resist distraction.

An estimated 7 million Americans now smoke marijuana with at least some frequency, according to government figures. And ever since pot smoking aroused nationwide concern four decades ago, mental health specialists have debated whether, or how much, brain damage might result from heavy use.

Although most experts have agreed that the drug is by no means "the devil's weed," leading inevitably to hopeless addiction and damaged brain cells, study after inconclusive study has indicated at least some mental impairment.

The newest research, being reported Wednesday in the Journal of the American Medical Association, is by far the most ambitious yet conducted. A team of eight American psychiatrists, psychologists and drug treatment experts with Connecticut's national Marijuana Treatment Project Research Group conducted the study.

It involved a battery of nine widely accepted psychological tests of brain function. All the data were analyzed by Nadia Solowij, a leading psychologist at Australia's National Drug and Alcohol Research Centre at the University of New South Wales in Sidney, who also wrote the report and helped design the project.

"These results confirm that long- term heavy cannabis users show impairments in memory and attention that endure beyond the period of intoxication and worsen with increasing years of regular cannabis use," the team concluded.

But Solowij cautioned strongly in an e-mail to The Chronicle that the study does not suggest in any way that pot causes serious brain damage.

"The long-term consequences of marijuana on memory function are not so tragic," she added. "Memory is impaired in very long-term heavy users, and that may affect their functioning in daily life," but the effects generally appeared to be modest.

The pot users in the study had been smoking an average of two joints a day for an average of 24 years and all had already enrolled in treatment programs for dependence in Seattle, Miami or Farmington, Conn. A small control group of volunteers had either never or rarely smoked pot or hadn't smoked in years.

The researchers gave the psychological tests to the heavy smokers an average of 17 hours after they had last smoked, although in some cases the time lag was only seven hours, while in others it was 10 days.

In her e-mail message, Solowij said the researchers had already tested the same group of subjects four months after they entirely stopped using the drug to see how long their memory functions were impaired.

"It is probably unlikely that the impairments would be permanent, but we just don't know that," she said.

Although Solowij said the research subjects were not dependent on any other drugs and were "generally fairly representative of long-term heavy cannabis users," a noted psychiatrist and marijuana expert at Harvard, Dr. Harrison Pope Jr., maintained that even moderate use of other drugs could be one of many "confounding variables" likely to make the study's conclusions less certain.

In an editorial in Wednesday's issue of the medical journal, and in comments Tuesday, Pope maintained that many of those variable could have affected the study conclusions: Some of the subjects might already have had impaired memory; some might have used other drugs now or in the past; and some might have been taking prescription medications or had psychiatric problems such as anxiety or depression.

"The safest thing to say at this point is that the jury is still out on the question of whether long- term marijuana use causes lasting impairment in brain function," Pope said.

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