GATEWAY TO NOWHERE?

The Evidence That Pot Doesn't Lead to Heroin.

Earlier this month, professor Yasmin Hurd of the Mount Sinai School of
Medicine released a study showing that rats exposed to the main
ingredient in marijuana during their adolescence showed a greater
sensitivity to heroin as adults. The wire lit up with articles
announcing confirmation for the "gateway theory"--the claim that
marijuana use leads to harder drugs.

It's a theory that has long seemed to make intuitive sense, but
remained unproven. The federal government's last National Survey on
Drug Use and Health, conducted in 2004, counted about 97 million
Americans who have tried marijuana, compared to 3 million who have
tried heroin (166,000 had used it in the previous month). That's not
much of a rush through the gateway. And a number of studies have
demonstrated that your chances of becoming an addict are higher if
addiction runs in your family, or if heroin is readily available in
your community, or if you're a risk-taker. These factors can account
for the total number of heroin addicts, which could make the gateway
theory superfluous.

On close inspection, Hurd's research, published in the journal
Neuropsychopharmacology, doesn't show otherwise. For the most part,
it's a blow to the gateway theory. To be sure, Hurd found that rats
who got high on pot as adolescents used more heroin once they were
addicted. But she found no evidence that they were more likely to
become addicted than the rats in the control group who'd never been
exposed to delta-9-tetrahydrocannabinol, or THC, marijuana's main ingredient.

Hurd began with two groups of rats. The first was administered THC
every three days during their early adolescence (beginning at 28 days
old) to approximate the sporadic marijuana use of American teens. The
second group was given no drugs. Then, at mid-adolescence (56 days),
both groups began a heroin regime. Hurd started by giving the rats a
low dose of the harder drug. None of them got hooked. So, she doubled
the fix. Each cage was equipped with an active and an inactive bar.
Depressing the active bar when a white light was on gave the rats a
hit of heroin; if they hit the bar regularly, that indicated
addiction. Rats in both groups hit the active bar at least twice as
often as they did the inactive one, which means they became addicted
at roughly the same rate.

The difference between the groups came post-addiction: For the first
15 heroin sessions, both sets used generally equal amounts of heroin.
Then the control rats leveled off. But the pot rats kept taking more
of the drug, leveling off at about a 25 percent higher dosage. This
increased use was evidence of their greater sensitivity to heroin.

Hurd says that because the marijuana-exposed rats demonstrated this
heightened sensitivity, she expected them to be more motivated in
pursuing the drug. But they weren't. The control rats paced their
cages and repeatedly pressed the active bars even when the light
indicating availability wasn't on. The pot rats, on the other hand,
figured out that the heroin was available only at certain times, and
that pacing and tapping the bar incessantly wasn't worth the trouble.
When heroin was available, the marijuana rats took more of it. But
when it wasn't, they chilled in the corner.

Extrapolate the study to human behavior, Hurd says, and it suggests
that teenagers who smoke pot are no more likely than other kids to
become addicted to heroin. (Her study doesn't speak to whether they'd
be more likely to try the drug.) If teens do get hooked on the hard
drug, though, they may develop a stronger addiction.

Hurd's results come on the heels of another marijuana finding that's
not what the drug's opponents want to hear. Donald Tashkin, a UCLA
medical with funding from the NIH's National Institute on Drug Abuse,
looked at more than 1,200 people with cancers typically associated
with cigarette smoking and a control group of more than 1,000 people
without cancer. To his surprise, he found no link between marijuana
and increased risk of cancer, even among the heaviest pot smokers. The
results of Tashkin's study, the largest of its kind that's been done,
will soon appear in the journal Cancer Epidemiology Biomarker and Prevention.

There are a couple of plausible explanations for Tashkin's finding.
Research finds that smoking less than a pack of cigarettes a day leads
to only a slightly higher cancer risk than not smoking at all. It's
two packs a day that triggers a much higher risk. Two packs is the
equivalent of 10 joints--more marijuana than almost anyone smokes.
Tashkin speculates that the risk threshold for pot might be too high
to measure in the United States. "One would have to repeat the study
in a society such as Jamaica," he says.

Another possibility, according to Tashkin, is that marijuana's
cancer-fighting elements and its carcinogens counteract each other.
Animal studies have shown that THC has an inhibitory effect on a
number of cancers. Marijuana also contains dozens of active
cannabinoids, several of which have been shown to block cancer cell
growth.

Tashkin's findings do not mean that marijuana is harmless. In previous
work, he has shown that pot smoke leads to chronic and acute
bronchitis at the same elevated rate as tobacco smoke. He is currently
studying the drug's relationship to pneumonia. But his latest results
about cancer risk, like Hurd's on the gateway theory, make pot seem
more rather than less benign. The federal government has announced the
results of Tashkin's past studies with press conferences and subway
ads. Don't look for any this time around.