TUESDAY, Jan. 21, 2020 (HealthDay News) -- Over 2 million Americans with heart disease have used marijuana, despite evidence that it might be harmful to them, a new research review finds.

The report, published in the Jan. 28 issue of the Journal of the American College of Cardiology, comes at a time when many states are legalizing medicinal or recreational marijuana use. And, some studies suggest, a growing number of Americans are lighting up -- or using the drug in other forms.

"We've reached an epidemiological shift," said Dr. Muthiah Vaduganathan, the senior author on the review. "For the first time, there are more marijuana users than cigarette smokers in the U.S."

But while the health consequences of tobacco smoking are well documented, less is known about the risks of marijuana smoking -- or other modes of delivery, like vaping and edibles.

Still, the existing evidence on marijuana and heart disease is "cause for concern," said Vaduganathan, a cardiologist at Brigham and Women's Hospital, in Boston.

"The evidence is modest because we don't have the [long-term] studies, or the breadth of research, that we have on cigarettes," Vaduganathan said. "But we do know that acute marijuana use induces increases in blood pressure and heart rate that could be problematic for people with heart disease."

Research has found that heart disease patients develop chest pain more readily if they smoke pot, while other studies suggest that marijuana use might even serve as a heart attack trigger, the review found.

It may also raise the risks of strokes and heart arrhythmias. In an Australian study of adults younger than 65, those who'd smoked pot at least once a week in the past year had a nearly fivefold higher risk of stroke than those who used it less often.

Meanwhile, a U.S. study found that of over 2.4 million marijuana users who'd been hospitalized, almost 3% had suffered an arrhythmia -- mainly atrial fibrillation, where the heart's upper chambers quiver instead of contracting normally. The prevalence of arrhythmias among marijuana users rose over time, between 2010 and 2014, though it's not clear why.

Any heart risks of marijuana would be most serious in people who have established heart disease. And that's not an uncommon scenario, based on data from a federal health survey.

Vaduganathan's team estimates that more than 2 million Americans with heart disease have used marijuana, either currently or in the past.

There are many unknowns, including how commonly people keep using marijuana after a heart disease diagnosis, and whether they think it's safe.

"I think a next step would be to conduct focus groups, to ask patients about current use and their perceptions of the risks," Vaduganathan said.

One concern is that marijuana may interact with the medications heart disease patients commonly use, including statins, certain blood pressure drugs, and the blood thinner warfarin.

"Medication levels in the blood would be anticipated to change," Vaduganathan said.

His advice to patients is to avoid marijuana use during "high-risk" periods, such as after a heart attack.

"And we discourage any use of synthetic cannabinoids or vaping in people with established heart disease," Vaduganathan added.

Synthetic cannabinoids -- which go by names like K2, Spice and Kronic -- are human-made chemicals similar to those found in marijuana plants. They can be much more potent than marijuana, Vaduganathan noted. Similarly, he said, vaping can boost the potency of marijuana -- and is not a "safer" alternative to smoking.

Dr. Salim Virani is a cardiologist with the Michael DeBakey VA Medical Center, in Houston, and chair of the American College of Cardiology's Prevention Section and Leadership Council.

"We don't have as strong an evidence base for marijuana and heart disease as we do for cigarette smoking," Virani said. "So there's a big need for research in this area. What are the risks? And what is the magnitude of the risks?"

In some cases, he said, doctors may want to screen patients for marijuana use -- such as people younger than 55 who have an unexplained heart attack.

Virani also encouraged heart disease patients to tell their doctors about any marijuana use.

"As doctors," he said, "we have to be open to having a discussion about it, and be clear about what we know and what we don't know."

More information

Harvard Medical School has more on marijuana and heart health.