VERY HEAVY POT USE CLOUDS MENTAL FUNCTION: STUDY

NEW YORK (Reuters Health) - People who smoked unusually large amounts of
marijuana performed worse on tests of mental function than their peers who
smoked less pot, even after a 30-day abstinence period, according to a new
report.

Heavy users performed worse on 69% of the 35 tasks than light users, though
their performances were not "clinically abnormal," the researchers found.
The 22 participants were admitted to hospital during the course of the
study and submitted to random urine tests to ensure they remained abstinent.

Lead author Dr. Karen Bolla characterized the study group as being
"unusual" because of the large number of joints they smoked per week. Heavy
users smoked on average 91 joints a week, or about 13 a day, while light
smokers smoked an average of 11 marijuana cigarettes a week.

Bolla, who is an associate professor of neurology and psychiatry at Johns
Hopkins University School of Medicine in Baltimore, Maryland, said the
results cannot be generalized to social smokers or those who use pot for
medicinal purposes, because they smoke far less marijuana. The potency
might also differ, she said.

"What this study shows is that marijuana can be neurotoxic if you smoke a
lot of it," Bolla told Reuters Health. She said this is particularly
concerning since the average age of study participants was 22 years old,
and the brain is still developing at that age. "You're putting a lot of
foreign stuff in there that we don't really know what it does to a
developing brain," she said.

The study, published in the November issue of the journal Neurology, found
that the mental functions most severely impacted were memory, executive
function (overall reasoning and functioning) and manual dexterity.

Bolla writes that these tasks in particular were affected because they are
controlled by the hippocampus, prefrontal cortex and cerebellum. These
brain areas are densely populated with cannabinoid receptors that attach to
THC, the active ingredient in marijuana.

In mice, excessive marijuana use might damage parts of the brain and "knock
out certain kinds of neurons," said Bolla. This can lead to receptors in
the brain being over-stimulated or under-stimulated, changing their
response to chemical messengers in the brain, similar to what might result
from a brain injury.

Marijuana is the most widely used illicit drug in the US. An estimated 7
million people use marijuana weekly, according to 2000 data from the US
Department of Health and Human Services.

This is only the second study to examine the residual effects of marijuana
use after more than a couple of days of abstinence. Dr. Harrison Pope Jr.,
a professor of psychiatry at Harvard Medical School and author of the other
study, found no difference in performance on cognitive tests between heavy
marijuana users and "control" subjects.

Pope said in an interview that his "hunch" was that the difference between
his results and Bolla's were the "sheer intensity" of marijuana use among
the participants in Bolla's study. Heavy users in Pope's study smoked on
average 1 or 1.5 joints over the course of a day.

According to Pope, people who smoke a lot of marijuana and start earlier
will do worse on tests of mental function. Whether the toxicity of the drug
itself is responsible, or factors like being in school less and being
unfamiliar with testing or being more impaired initially and turning to pot
for that reason, is difficult to know, he added.

SOURCE: Neurology 2002;59:1337-1343.