There’s more evidence that smoking marijuana can be dangerous for people with heart disease, according to two new studies presented recently at the American Heart Association’s Scientific Sessions. But in an unexpected twist, people who use cannabis were less likely to experience sudden kidney failure, the researchers found.

Patients who smoked marijuana and underwent angioplasty to clear blocked arteries were more likely to experience stroke and bleeding after the nonsurgical procedure than those who didn’t use pot, one study found. The second study concluded that patients who had survived a heart attack and used marijuana were more likely than those who did not use cannabis to have a subsequent heart attack. Both studies were released Monday.

“Marijuana is becoming more accessible, and patients should be aware of the increased risk after [angioplasty],” said the lead author of the study, Dr. Sang Gune Yoo, an internal medicine resident at the University of Michigan. “While these are risks to be aware of, they shouldn’t deter patients from obtaining this lifesaving procedure.”

In fact, four more states legalized recreational marijuana during the 2020 election, bringing the total to 15 plus the District of Columbia. Moreover, 34 states, plus D.C., have made medical marijuana legal.

The new findings are another example of why we need more studies into the effects of cannabis on the health of the heart and the rest of the body, Yoo said, noting that its classification by the federal government as a Schedule 1 drug has hindered scientific research.

The research is especially relevant for older Americans. Heart disease is the No. 1 killer in the U.S. and the Centers for Disease Control and Prevention estimates that each year approximately 805,000 Americans have a heart attack. The majority, or 605,000, are first-time heart attacks. Each year hundreds of thousands of patients in the U.S. have coronary angioplasty — a procedure for blocked arteries that improves blood flow to the heart, according to the American Heart Association. Many of those patients also receive stents, a small, wire mesh tube which helps keep the artery open.

Feb. 25, 202002:19

To take a closer look at the impact of marijuana on angioplasty outcomes, Yoo and his colleagues examined data on 113,477 Michigan patients, 3,970 of whom self-identified as marijuana users. After matching 3,903 users with 3,903 nonusers, the researchers found that more pot smokers experienced bleeding (5.2 percent vs. 3.4 percent) and strokes (0.3 percent vs. 0.1 percent).

An intriguing finding that the authors couldn’t explain was that the marijuana users were less likely to experience sudden kidney failure.

Is it THC or the smoke?

In the other study, which analyzed information from a national database, researchers found that among patients who had an artery-clearing procedure after a heart attack, those who used marijuana had a higher rate of subsequent heart attacks than those who don’t use cannabis, or 7.2 percent vs. 4.5 percent. This study also had an intriguing finding: Heart attack risk factors — including hypertension, diabetes and high cholesterol — were significantly lower in cannabis users.

Experts contacted by NBC News had mixed reactions to the new reports.

“I have spent the last 25 years studying the effects of marijuana and THC [the psychoactive component in cannabis], and I think the Yoo study raises some important questions, especially since we’ve seen more and more reports of cardiovascular events occurring in the context of marijuana,” said Dr. Deepak Cyril D’Souza, a professor of psychiatry at the Yale University School of Medicine. “This is an interesting paper.”

One question the study can’t answer is whether the increased risks come from the marijuana, the THC in the marijuana or from the smoke that comes from burning a plant-based product, D’Souza said.

“In our studies on THC, we found a very robust increase in heart rate and an effect on blood pressure that can be quite profound,” D’Souza said, adding that blood pressure rose in people who were lying down. “Our studies typically included people who were healthy and young. Extrapolating to someone who is older and has cardiovascular issues or other medical problems, it could be problematic if they were to use marijuana.”

Looking at the totality of the two studies, they seem to contain some contradictory findings, said Dr. Peter Grinspoon, a cannabis expert, instructor at Harvard Medical School and board member of Doctors for Cannabis Regulation. On the one hand, Grinspoon said, researchers are reporting increases in stroke, bleeding and second heart attacks in those using cannabis, but on the other, they are reporting improvements in cardiovascular risk factors, such as hypertension, high cholesterol and diabetes.

Grinspoon was particularly struck by the lower rate of kidney failure among those who used marijuana in the Yoo study.

“By their measures, they’ve actually shown that cannabis decreases kidney damage, which should be a huge headline in itself,” he said.


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