Using marijuana as little as once per month is associated with a higher risk of both heart attack and stroke, according to a large study published Wednesday by researchers from Massachusetts General Hospital. The risks rose sharply the more frequently marijuana was used. 

The paper, which was published in the Journal of the American Heart Association, adds to the growing body of evidence suggesting marijuana may be harmful to the cardiovascular system. 

Scientists analyzed data on nearly 435,000 patients, ages 18 to 74, to see whether there was a link between marijuana use and a higher risk of heart disease, stroke or heart attack. The data came from a behavioral risk factor survey collected from 2016 to 2020 by the Centers for Disease Control and Prevention. 

Compared with people who had never used marijuana, daily cannabis users had 25% higher likelihood of heart attacks and 42% higher risk of strokes. People who used marijuana just once a week had a 3% increased likelihood of a heart attacks and 5% higher risk of strokes during the study time frame.

The study is among the largest to show a connection between marijuana use and cardiovascular health in people who don’t also smoke tobacco, said lead researcher Abra Jeffers, a data scientist at Massachusetts General Hospital. 

Nearly 75% of people in the study reported smoking as the most common way they got high. They also consumed edibles and vaped. The study did not specifically look at the risks of smoking marijuana compared to edibles.  

It’s unclear from the paper whether marijuana directly causes heart attacks and strokes or whether people who are already at risk are more likely to use it. 

Historically, some have dismissed studies looking at marijuana and heart problems because participants often use both tobacco and marijuana products, making it hard to determine which substance is really to blame, Jeffers said.

Robert Page, a clinical pharmacist who specializes in heart disease at the University of Colorado Skaggs School of Pharmacy, is worried about the emerging connections between marijuana consumption and the heart. Page was the lead author of a comprehensive statement on cannabis released by the American Heart Association in 2020.

“I think we’re beginning to see the same things we saw with smoking cigarettes back in the ’50s and ’60s — that this is a signal,” Page said. “I feel like we’re repeating history.”

Ultimately, it will take more rigorous studies to draw any firm conclusion, he said, which would involve following people for years and monitoring their marijuana use. That type of research is difficult to conduct because marijuana is still a Schedule 1 substance under the Controlled Substances Act. 

What if I just use marijuana occasionally? 

The new research found that the risks of heart attacks and strokes became higher the more days per month people used marijuana, which is called a “dose-response relationship.”

“If something is really bad or a toxin, you’d expect more of it to be worse,” said Dr. Deepak Bhatt, the director of Mount Sinai Fuster Heart Hospital in New York, who was not involved with the research. “The fact that there’s a dose response makes it seem like it probably is, in fact, the cannabis that is causing the bad outcome.”

The president of the American Heart Association, Dr. Joseph Wu, the director of the Stanford Cardiovascular Institute, drew a comparison to other common substances. 

“It’s the same dose response as somebody who smoked tobacco or as somebody who drinks alcohol,” he said. “The more you drink, the more problems you are going to have, because these are toxins.”

Ultimately, the researchers concluded that the people who really should be avoiding marijuana smoking altogether are those with pre-existing heart disease, estimated at 1 in 20 Americans. 

That marijuana is associated with heart problems is a very urgent message for Americans to be aware of, Wu said, as 1 in 5 people over age 12 now report having used marijuana in the last year, according to the National Survey on Drug Use and Health. 

“Just because something’s been legalized doesn’t mean it’s safe,” he said.

Are edibles safer?

Smoking was the most common way cannabis was consumed in the new paper, although edibles are not necessarily safe, either.

“If you force me to answer I would say not smoking is a better way of consuming it,” Bhatt said. “When you smoke things, that makes them more toxic, but that doesn’t mean that we can say it’s definitely safe to consume it as an edible.”

Laboratory studies have shown that THC, the psychoactive ingredient in marijuana, can cause an increase in inflammation in the blood vessels, so edibles aren’t necessarily risk-free, Wu said.

“If you’re smoking marijuana it’s probably doing double the damage compared to just using edibles,” Wu said. “When you eat the edible, the THC goes into your body and can cause vascular inflammation. Whereas when you smoke, there is damage from the particulate matter and then the THC gets absorbed into your body, as well.” 

It’s not yet known why smoking marijuana affects the cardiovascular system, but there are a few possibilities, Bhatt said.  

A phenomenon called oxidative stress, an imbalance between free radicals and antioxidants in the body, can cause inflammation and damage to blood vessels. Other reasons could include marijuana’s triggering abnormal heart rhythms or even activating platelets, cells in the body that can make blood more likely to clot, leading to a heart attack or stroke. 

Should young healthy people be concerned?

The paper found that among younger adults, defined as men younger than 55 and women younger than 65, cannabis use was significantly associated with 36% higher combined odds of coronary heart disease, heart attack and stroke, regardless of whether or not they also used traditional tobacco products. 

“I’ve seen it through the years with clinical practice many times where sometimes we bang our heads thinking, ‘Why [is] this person in their 20s, or 30s or 40s [coming] in with a heart attack?” Bhatt said. While it can often be attributed to things like extremely high cholesterol or cocaine use, he said, sometimes there’s only one factor they have in common. 

“The only thing I can find after asking and asking again and again in terms of potential risk factors is marijuana,” he said. “So the smart thing to do would be not to smoke marijuana, but I realize it’s extremely popular and that’s advice that may not be well received by all.”