Marijuana use is on the rise as more states legalize it for medicinal and recreational purposes, and physicians are fielding more questions about its safety.

Although smoking tobacco is responsible for approximately one in four deaths from cardiovascular disease, the effects of smoking marijuana on the heart are not fully understood. Some studies suggest that marijuana can trigger heart attacks and strokes in some users.

Ersilia DeFilippis, MD, a second-year cardiology fellow at Columbia University Irving Medical Center and NewYork-Presbyterian, first became interested in marijuana’s effect on the heart a few years ago when studying heart attacks in people under 50. “We noted that 10% of patients in a registry of young heart attack patients had used marijuana and/or cocaine,” she says.

DeFilippis and colleagues recently reviewed the medical literature to find out what’s known about marijuana’s effect on the heart and what’s still unknown. Their full report was published January 20 in the Journal of the American College of Cardiology.

Here are five highlights from the review:

2 million people with heart disease have used marijuana

Marijuana is the most commonly used drug of abuse. It’s estimated that approximately 90 million American adults have used the drug at least once in their life, and more than 39 million have used the drug in the past year.

Based on responses to the National Health and Nutrition Examination Survey from 2016, DeFilippis and her colleagues estimate that about 2 million adults in the United States who have cardiovascular disease currently use marijuana or have used the drug in the past. 

“In addition to the 2 million marijuana users with diagnosed cardiovascular disease, many more may be at risk,” DeFilippis says. “With many adolescents and young adults turning to marijuana, it is important to understand the cardiovascular implications they may face years down the line.”

Marijuana’s potency today Is higher

The potency of marijuana—the percentage of THC contained in the plant—has steadily increased over the past 30 years, from about 4% in the mid-1990s to 12% in 2014. However, most scientific studies of cannabis tested products with THC levels between 1.5% and 4%.

“Higher potency may translate into greater effects on the conduction system, the vasculature, and the muscle of the heart,” DeFilippis says. “It also highlights the need for real-world data given the variety of marijuana products and formulations available for purchase.”

THC is the most psychoactive chemical in marijuana, but marijuana also contains more than 100 compounds, called cannabinoids, that are chemically related to THC. 

Receptors for cannabinoids are highly concentrated in the nervous system but also can be found in blood cells, muscle cells, and other tissues and organs. 

Cannabinoids can interact with drugs used to treat heart disease

Cannabinoids inhibit certain enzymes in the body, which affects the metabolism of many drugs for heart disease, including antiarrhythmics, statins, calcium-channel blockers, beta blockers, and warfarin.

Researchers believe that cannabinoids may increase the activity of these prescribed drugs in the body, though limited data are available to guide physicians in adjusting dose to compensate for marijuana use.

Marijuana may be linked to heart attacks and strokes

Studies have identified marijuana smoking as a potential trigger of heart attacks, and marijuana use is not infrequently detected in adults who have experienced heart attacks at an early age (under 50).

A small experimental study found that smoking marijuana can bring on angina (chest pain) more quickly in patients with coronary heart disease compared with smoking a placebo.

Though current evidence for a link between marijuana and heart attacks is modest, it’s thought that smoking marijuana may increase cellular stress and inflammation, which are known to be precipitating factors for coronary artery disease and heart attacks.

Cerebrovascular events, including strokes, also have been associated with marijuana use. It’s thought that marijuana may induce changes in the inner lining of blood vessels or alter blood flow.

Physicians should screen for marijuana use

“Although we need more data, the evidence we do have indicates that marijuana use has been associated with coronary artery disease, arrhythmia, cardiomyopathy, and more,” DeFilippis says.

“Therefore, asking patients about marijuana use may help in risk assessment. In addition, we know that marijuana use affects the metabolism of many common cardiac drugs. In order to make sure patients are getting therapeutic doses without untoward side effects, it is important for cardiologists to talk to their patients about marijuana use.