Marijuana may boost heart attack risk.

Researchers have recorded several instances in which otherwise healthy people have suffered heart attacks shortly after smoking marijuana, the most commonly used illegal drug in the United States. Experiments have also linked marijuana use to elevated blood pressure and decreased oxygen supply to the heart muscle. A large study now suggests that marijuana indeed puts a person at increased risk of immediate heart attack.

Researchers interviewed 3,882 people a few days after they had survived a heart attack. The researchers found that 124 had smoked marijuana in the past year, 9 within 1 hour of the attack.

By using interview responses to estimate how often these people smoked marijuana, researchers compared heart attack risk for each of these nine people during the hour after smoking pot with other times when they hadn't smoked.

A 50-year-old man has roughly a one in a million chance of having a heart attack in any given hour, says study coauthor Murray A. Mittleman of Beth Israel Deaconess Medical Center and Harvard Medical School in Boston. The data suggest that within an hour of smoking pot, that risk rises to 5 in a million, he says.

"It's a rare exposure," Mittleman says, and one that's influenced by other factors such as obesity, cigarette smoking, and stress. Three of the nine people who had a heart attack within 1 hour of smoking pot also had taken cocaine, had sex, or both. After excluding these people from the calculations, marijuana smoking within the hour still coincided with a tripled risk, the researchers report in the June 12 CIRCULATION.