HEAVY POT USE IMPAIRS BRAIN, NEW STUDY FINDS BUT SCIENTISTS UNSURE
ABOUT EXTENT OF HARM

A three-city study of heavy marijuana users has found that long-term
pot smoking impairs brain function, scientists report today.

In an elaborate study of more than 150 men and women being treated
for dependence on the 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 today 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 today's issue of the medical journal, and in
comments yesterday, 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.