Research into the addictive qualities of marijuana has always been controversial. In the 1970s, psychiatrists documented flu-like symptoms in heavy cannabis users withdrawing from the drug. But the symptoms were so mild compared with the agony of withdrawal from heroin, cocaine or nicotine that many doctors dismissed the studies as inconsequential. Others charged that government researchers were attempting to demonize what was a mostly harmless drug.
"But there's stuff out there now that's 10, 20, even 50 times as potent we could get for research in the '70s," said Dr. Reese Jones, a professor of psychiatry at UC San Francisco. "It's like studying the effects of high doses of alcohol using 3.2% beer. Now, marijuana is more analogous to 100-proof vodka. Not every kid's getting that, but the ones who do and come into treatment will get sick when they go off the drug. And when you give them marijuana, they feel better."
Doctors only vaguely understand how marijuana affects the body. It can act as a stimulant or depressant. It eases pain, as opiates do, but it can also increase anxiety and induce paranoia. Its most psychoactive ingredient, tetrahydrocannabinol, or THC, acts throughout the brain, and the plant contains hundreds of other chemicals whose effects are unknown.
Using brain-imaging technology, scientists have shown in recent years that THC is especially active in the cerebellum, which helps regulate movement, and in the frontal cortex, the part of the brain that enables us to make judgments and inhibit impulses. "These findings can help explain how chronic marijuana use causes some behavior changes -- such as why intoxication can lead to automobile accidents," said Volkow, who did some of the imaging research.
Steven Sussman, a professor of preventive medicine at USC, began tracking a group of 339 teenage marijuana smokers in the late 1990s. All the young men and women were heavy users when the study began. Five years later, 42% have quit and 58% still smoke frequently, Sussman reports in a paper due out later this year. The difference between the two groups is partly social: The quitters were more likely to have gotten married than the others and had fewer marijuana-using friends throughout the study. But those who managed to quit also tended to use less than their peers from the beginning.
In short, dosage matters. And if frequent marijuana users are getting more THC, doctors say, then it's time old assumptions about the harmlessness of the drug were reexamined.