Marijuana ingredient slows artery hardening.

A new animal study suggests that low doses of the chemical that causes marijuana's high may halt the progression of atherosclerosis, a disease that narrows and hardens blood vessels.

Atherosclerosis starts when a vessel's inner wall becomes damaged. Cellular debris builds up into plaques, and immune cells called leukocytes inflame the damaged area. Eventually, this combination can completely block the blood vessel, causing a heart attack or stroke.

Previous research suggested that tetrahydrocannabinol (THC), the psychoactive component of marijuana, has anti-inflammatory properties. To determine whether THC could affect atherosclerosis, Francois Mach of Geneva University Hospital in Switzerland and his colleagues tested the chemical on mice genetically predisposed to develop a version of the disease.

The mice ate a high-cholesterol diet for 11 weeks, which facilitated the buildup of thick plaques within their blood vessels. For the last 6 weeks of the diet, Mach's team gave some of the mice daily oral doses of THC.

The scientists found that the mice given a minute amount of THC had significantly smaller plaques and less inflammation than did those that hadn't received the chemical or that got larger doses. The only effective dose was the mouse equivalent to about one-tenth of a marijuana joint, considerably less than what creates a high.

Mach says that the results, published in the April 7 Nature, don't mean that smoking pot can reduce atheroselerosis in people. Since only low doses had a therapeutic effect in mice, the dose from smoked pot is probably too high to be beneficial. Furthermore, Mach notes that marijuana smoke injures the lungs, and so may promote atherosclerosis.--C.B.