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.