Pain relief is emerging as a potential use for marijuana—although one new study published in the journal Anesthesiology found that too strong a dose can actually worsen pain.

California researchers gave 15 volunteers marijuana cigarettes with small, medium, and large doses of tetrahydrocannabinol (THC), the herb’s main active ingredient. They then inflicted mild pain by injecting the volunteers with capsaicin, the substance that gives chili peppers their bite. Volunteers who smoked the medium dose—4 percent THC—reported less pain than those who got a lower dose or a placebo. But higher doses seemed to increase pain sensitivity.

Another trial of 50 patients with HIV-associated nerve pain, published in Neurology, found that a moderate marijuana dose relieved burning, aching, and nerve pain as effectively as the oral pain drugs typically prescribed. More than half of those who smoked the drug achieved a significant decrease in pain, compared with a quarter of those in the placebo group.

Long-term use of marijuana can expose users to a significant risk of lung damage. In patients with limited life expectancy, such as those with advanced AIDS and terminal cancer, the benefits of smoking modest doses of marijuana may outweigh the risks. Twelve states now allow the medical use of marijuana: Alaska, California, Colorado, Hawaii, Maine, Montana, Nevada, New Mexico, Oregon, Rhode Island, Vermont, and Washington. It remains illegal under federal law in the U.S.

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