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THE question of whether marijuana is physically addictive has been troubling
researchers for years—but the latest evidence suggests that lab monkeys,
at least, are easily hooked.

For the first time, scientists in the US have shown that monkeys will seek
out the active chemical in cannabis just as they would with cocaine or morphine.
The finding has created an uproar over the implications for humans who smoke
pot.

Therapists and psychiatrists who treat marijuana dependency are convinced the
study will persuade people to take pot’s potential for abuse more seriously. “A
lot of people think it’s not addictive,” says Ron Kadden of the University of
Connecticut Health Center. “[Users] have been told by treatment professionals
and friends that they couldn’t really be addicted to marijuana.” But Kadden says
he recently found plenty of takers when he advertised a programme tailored to
treat cannabis dependency.

Campaigners for the legalisation of dope, on the other hand, have called the
study “pseudoscience”, designed to fulfil a political agenda. “We take umbrage
with the government’s seemingly never-ending penchant to prove that cannabis is
harmful enough to justify the 70-year prohibition,” says Allen St Pierre of the
National Organization for the Reform of Marijuana Laws in Washington DC.

Last week, a survey showed that one in ten people in England and Wales has
used cannabis in the past year. The report from the European Monitoring Centre
for Drugs and Drug Addiction says that cannabis is regularly used by 45 million
Europeans.

However, proving that marijuana is addictive like cocaine or even nicotine
has been difficult. For years, scientists have been unable to get animals to
give themselves THC, the active chemical in marijuana.

In the latest attempt, Steven Goldberg and his colleagues at the National
Institute on Drug Abuse in Baltimore, Maryland, gave squirrel monkeys a small
injection of THC every time the animals pressed a lever. The monkeys quickly
learned to give themselves as many as thirty injections of the drug in a
one-hour session.

Giving the animals a drug that blocks the brain’s cannabinoid receptor,
however, stopped the monkeys pressing the lever. “We’re interested in looking at
potential compounds that might be used as medications for drug abuse,” says
Goldberg. “Now that we have a model of how THC supports drug behaviour we can go
in and try to intervene.”

The researchers say they know why their experiments succeeded where others
had failed. They gave the monkeys as much THC as you would get from a single
puff on a joint—with the dose adjusted for the animals’ body weight. This
meant the monkeys took doses of THC that were five-fold less than scientists
have tried in the past.

People who have trouble quitting may take comfort from the study, says Elena
Kouri of Harvard Medical School. She has studied withdrawal symptoms like
tremors in people who have smoked pot daily for years. “If we educate them that
it’s not that they don’t have the willpower, that it’s the drug, I think it will
help them go through the withdrawal.”