BOSTON--In the first hour after smoking marijuana, a person's risk of a heart attack can shoot up nearly five-fold, say researchers at Beth Israel Deaconess Medical Center in Boston. The study is believed to be the first to document a link between smoking marijuana and heart attacks.
As an immediate trigger for heart attack, marijuana is less risky than a spurt of exercise for a couch potato or a snort of cocaine. But marijuana smoke is about twice as dangerous as exercise for a fit person, sex for a sedentary person, a tantrum of anger or a bout of anxiety.
"As baby boomers enter the age at which coronary disease is the most important cause of death and the leading cause of morbidity, we may see an increase in marijuana-associated heart attacks," says internist Murray Mittleman, M.D., director of cardiovascular epidemiology at BIDMC. "In the debate about medicinal use, marijuana may be contraindicated for people with existing coronary disease."
Mittleman will present his results Thursday, March 2, at the Conference on Cardiovascular Disease Epidemiology and Prevention in San Diego. The meeting is sponsored by the American Heart Association.
The findings are the latest to emerge from a multicenter study of 3,882 patients who survived heart attacks. In this report, 124 people reported using marijuana regularly. Of these, 37 people reported using marijuana within 24 hours of their heart attacks, and nine smoked marijuana within an hour of their heart attacks. After smoking marijuana, the researchers found that the relative risk of a heart attack increased by 4.8 times within the first hour but dropped to 1.7 times in the second hour, suggesting a rapid decline in the acute cardiac effects.
Despite the surprisingly high percentage of people under age 50 who reported current marijuana use in the study -- 12.5 percent -- Mittleman doesn't anticipate an epidemic of marijuana-triggered heart attacks. "For a 50 year old without any other risk factors, this translates to a change in absolute risk of a 10-in-a-million chance of having a heart attack in the first hour after smoking marijuana," says Mittleman, who also holds a doctorate in epidemiology and teaches epidemiologic methods at the Harvard School of Public Health.
Mittleman can't explain exactly how marijuana may trigger a heart attack. Mechanisms could include the active ingredient THC or merely the smoke from burning plants, he says. "Smoked marijuana or oral THC increases the heart rate by about 40 beats per minute," Mittleman says. "It causes the blood pressure to increase when the person is lying down and then abruptly fall when the person stands up. This could precipitate a heart attack. On the other hand, except for THC in marijuana and nicotine in cigarettes, the smoke constituents from plant combustion are similar. The immediate effects of inhaling these substances is not well-understood."
Over the past six years, the multicenter study has revealed other immediate triggers of heart attacks with varying relative risks: cocaine, exercise, sex and psychological states. The study uses an innovative case-crossover design devised by Mittleman and his colleagues at BIDMC, the Harvard School of Public Health and Massachusetts General Hospital that compares acute exposure to a potential heart attack risk to another time when the person is not exposed to the risk.
The study was funded by the National Heart, Lung, and Blood Institute and the American Heart Association.
Beth Israel Deaconess Medical Center is a major patient care, research and teaching affiliate of Harvard Medical School and a founding member of CareGroup Healthcare System. BIDMC is the third largest recipient of National Institutes of Health research funding among independent U.S. teaching hospitals.
NOTE: Mittleman is available for comment on Monday, February 28 until 1:00 p.m. EST and on Tuesday, February 29 after 1:00 p.m. EST.