CON: MARIJUANA'S DAMAGING EFFECTS

Scientists Say Pot's Health-Related Risks Are Real but Small in Some Instances.

Marijuana is the most widely used illicit drug in the country -- an
estimated 25 million Americans smoked it within the last year and
close to 100 million have smoked it at least once in their life,
according to the most recent National Survey on Drug Use and Health
by the federal Substance Abuse and Mental Health Services Administration.

Rates and severity of marijuana addiction pale in comparison to that
of legal addictive drugs, alcohol and nicotine, according to the
Advisory Council on the Misuse of Drugs, a panel of independent
experts advising the British government, in a rare head-to-head,
scientific comparison.

Yet, the fact is, recreational use can lead to addiction, and
inhaling marijuana smoke is unhealthful for the lungs. Some
researchers argue that marijuana may predispose heavy users to mental
illnesses such as psychosis and depression.

How big are these risks and how should they be measured against
health benefits? "The FDA has ruled that marijuana has no medical
benefits, but its harms are well known and proven," says Tom Riley, a
spokesman for the White House Office of National Drug Control Policy,
referring to an April 2006 statement released by the FDA and several
other federal agencies concluding that smoking marijuana was not of
medicinal use.

For comparison's sake, Riley cites the prescription drug Vioxx. The
FDA, he notes, pulled Vioxx off the market in spite of its proven
efficacy, because it created problems in a small number of people.

Then, too, the number of people adversely affected by marijuana use
is large, Riley says. "There are more teens in drug treatment for
marijuana dependence than for alcohol or any other drug," he says.

Marijuana is a Schedule 1 drug by the Drug Enforcement
Administration's Controlled Substances Act, a classification reserved
for drugs carrying the highest risk for addiction and no medical benefit.

Scientists have reviewed the weed's risks and find them to be real,
but small. Ten years ago, the Institute of Medicine reviewed the
scientific evidence about marijuana at the request of the Office of
National Drug Control Policy. The 1999 report states that, "except
for the harms associated with smoking, the adverse effects of
marijuana use are within the range of effects tolerated for other medications."

In February, the American College of Physicians, the nation's
second-largest physicians group, released a position paper in support
of medical-marijuana research, protecting doctors from criminal
prosecution and rescheduling marijuana as a less harmful drug.

A British advisory group this year found no evidence to reclassify
cannabis as a more harmful drug in that country. In contrast to the
U.S., the U.K. puts cannabis in the lowest category (Class C) in
terms of criminal penalties for possession or sale, although
government officials are campaigning to move it to Class B.

To investigate the risks of marijuana, researchers typically use
heavy marijuana smokers as subjects. Though such a study design may
be convenient, it makes interpretation tricky because heavy users may
have traits in common besides smoking pot. Thus, says psychologist
and marijuana researcher Stanley Zammit of Cardiff University in
Wales, it is not easy in these kinds of studies to separate out the
contribution of marijuana to any measurable effect in the group.

Psychosis

Claims of a link between marijuana use and psychotic episodes came
under scrutiny after the U.K. downgraded cannabis from Class B to
Class C in 2004. In 2007, Zammit was asked by England's Department of
Health to survey the existing evidence to determine the long-term
risks for mental illness from using cannabis. After researching the
literature and including only those studies that satisfied certain
criteria, he combined the results in a 2007 Lancet paper.

He concluded that marijuana use was associated with an increased risk
of psychosis -- ranging from self-reported symptoms such as delusions
or hallucinations to clinically diagnosed schizophrenia.

The risk is small, he adds. Cannabis use was associated with a 40%
increase in risk overall and up to a twofold increase in heavy users.
Because the risk of any person developing psychosis in their lifetime
is about 2% to 3%, cannabis use at worst increases that to 5%. "So
95% of the people are not going to get psychotic, even if they smoke
on a daily basis," Zammit says.

Zammit adds that "the main limitations of these studies is that you
can never be sure that it's the cannabis itself that's causing this
risk." Heavy users of marijuana may differ from nonusers in other
traits -- including those that lead independently to increased drug
use and risk of psychosis. The studies he reviewed tried to take into
account this possibility but could not rule it out entirely.

The bottom line? "The evidence is probably strong enough that people
should be aware of this risk," he says.

Even if it's real, the risk of developing psychosis because of
marijuana use is smaller than with use of some other drugs --
including legal ones such as cigarettes, says Mitch Earleywine, a
psychologist at the State University of New York University at Albany.

Grant says that numbers of schizophrenia cases have not increased
since before the 1960s, when widespread marijuana use began. "The
data are variable to be sure, but most studies have found that over
the years the rate of schizophrenia has been stable or even
declining," he says.

Depression

In an American Journal of Psychiatry study, 1,920 adults were
assessed for marijuana use and depression and followed for 15 years.
In those subjects who had no depressive symptoms at the study's
start, marijuana abusers were four times more likely to develop
depressive symptoms down the road. But Zammit, who reviewed this
paper and 23 others in his 2007 Lancet paper, says the data overall
are even murkier than for psychosis. Most of the studies he reviewed
did not assess symptoms of depression before marijuana use, and so
didn't rule out the idea that depression makes someone more likely to
smoke marijuana -- and not the other way around.

Thinking

A review of the scientific literature published in the Journal of the
International Neuropsychological Society in 2003 looked at whether
marijuana smoking had lasting effects on cognition after THC has left
the body. Marijuana use was found to have small effects on memory in
long-term users -- measured by asking subjects to recall words, for
instance -- but no differences were seen on attention, verbal skills
and reaction time. "We were actually surprised," says Grant, an
author on the study. Even if the marijuana itself wasn't causing such
things, he expected marijuana users might have other
less-than-healthful behaviors -- they may drink a bit more, or use
some other drugs, and "you might expect them to do a little worse."

A 2002 study published in the Journal of the American Medical Assn.
found that a group of 51 heavy marijuana users (two joints per day)
recalled two to three fewer words on average than nonusers in a
memory test with a list of 15 words.

A second study, published in the Archives of General Psychiatry in
2001, found a similar deficit in 63 daily marijuana smokers who
hadn't smoked for up to a week. After 28 days of not smoking
marijuana the effect disappeared.

Children

Studies on brain function and mental illness cited above were
conducted in adult marijuana users. How the drug affects adolescents
is not completely resolved, but the data are more troubling.

A 2000 paper in the Journal of Addictive Diseases recruited 58
marijuana users and found structural changes in the brains of those
who had starting smoking marijuana before age 17 but not in those who
didn't start smoking until they were older.

"There's also a modest decrease in IQ if teens use heavily, though
weekly users and folks who quit don't seem to show it," Earleywine
says. Adolescence, he says, is a time when brain neurons are making
oodles of new connections, and it's possible that a psychoactive drug
such as marijuana may adversely influence that process.

Lungs

Before it has any effect on the brain, marijuana smoke enters the
body through the lungs. Dr. Donald Tashkin, professor of medicine at
the UCLA David Geffen School of Medicine, has studied the pulmonary
consequences of marijuana use for 25 years, recruiting a group of 280
heavy habitual pot smokers in the early 1980s, including some who
also smoked cigarettes. (Subjects averaged three joints per day for
an average of 15 years.) For comparison, he also recruited cigarette
smokers who didn't use marijuana and people who didn't smoke anything.

Tashkin has done a number of studies over the decades comparing these
groups. "I began with the hypothesis that regular smoking of
marijuana would have an impact on the lungs qualitatively similar to
the impact of regular tobacco smoking," he says. That's because the
smoke of both plants are more similar than different.

Tashkin and his colleagues did find symptoms of chronic bronchitis in
his marijuana-smoking group. In a 1987 study in the American Review
of Respiratory Diseases, they reported that incidence of chronic
cough, sputum production and wheezing was similar to that in cigarette smokers.

In a second study in the same subjects published in the American
Journal of Respiratory and Critical Care Medicine in 1998,
examination of the airways and the cells lining the airways found
swelling, redness and increased secretions in marijuana users.
Biopsies showed "extensive, widespread damage to the mucosa," Tashkin
says, similar to what was seen in tobacco users. "This is amazing,
because the marijuana smokers average three joints a day, but the
tobacco controls smoked 22 cigarettes, suggesting that on a
cigarette-to-cigarette basis, marijuana may be more damaging."

But marijuana smokers differ from tobacco smokers in other,
potentially more important ways, Tashkin adds. They do not seem to
develop more serious consequences of cigarette smoking, namely
chronic obstructive pulmonary disease (COPD) -- the fourth leading
cause of death in the U.S., killing 130,000 people each year -- or
lung cancer, the most common cancer in Americans and responsible for
an additional 160,000 annual deaths, according to 2005 statistics
from the Centers for Disease Control and Prevention.

To study lung cancer, Tashkin looked at more than 600 lung cancer
patients and more than 1,000 control patients matched for age,
socioeconomic class, family history and other alcohol and drug use
(along with many other potential influences).

The results, published in a 2006 paper in Cancer Epidemiology
Biomarkers and Prevention, found a large number of regular marijuana
smokers were present in both groups, but statistically there were no
more in the cancer group than control group, suggesting no
association between marijuana use and lung cancer. Tobacco smokers,
on the other hand, showed a dose-dependent increase in risk: with a
30%, 800% and 2,100% increased risk of lung cancer in those who
smoked less than a pack, one to two packs or more than two packs per
day, respectively.

Other studies have found increased cancer risk. A study of 79 lung
cancer patients and 300 controls published in the European
Respiratory Journal this year found a fivefold increased risk in the
heaviest marijuana users (daily use for 10 years) and no effect in
less heavy users.

But Tashkin says this conflicting report was much smaller in scale,
having fewer than 20 subjects in the group of heaviest marijuana
users. "My critique would be: It's a small study. I think that their
small sample size is responsible for vastly inflated estimates," he says.