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About 2 million American adults with cardiovascular disease say they’ve used marijuana, despite the drug being a potential risk for heart health, a new research says.

The research, published in Monday’s Journal of the American College of Cardiology, is a review of previously published studies.

Observational studies used in this review suggest there is an association between marijuana use and a range of heart problems. There are few gold-standard style studies, known as randomized control trials, that look at this connection.

Some scientists believe smoking marijuana can cause some of the same cardiovascular health problems that are caused by smoking cigarettes, according to these observational studies . Tests show that while the dominant substance produced in tobacco and weed is different when smoked, some of the same cardiotoxic chemicals are produced.

People who smoke weed typically take fewer puffs than people who smoke cigarettes, but they hold the smoke in longer which may deliver more cardiotoxic chemicals, studies have shown.

The report found 36 studies that showed the top three triggers for a heart attack included the use of cocaine, eating a heavy meal and smoking marijuana. Smoking weed can cause an increase in a user’s heart rate and blood pressure and, while marijuana can be consumed through edibles, 77.5% of people surveyed said they smoked it.

Some studies have shown that using weed can contribute to weight gain, which is another stressor on the heart, although other studies have contradicted those findings.

Currently, there are no guidelines addressing any association between marijuana and cardiovascular disease. The 2017 National Academies of Sciences, Engineering, and Medicine released a report looking at whether there is a connection, but concluded the evidence was unclear.

The authors urge scientists to do more research on this topic.

In the meantime, they advise cardiologists to screen patients for use of the drug and to talk to patients about how much they use and how they use it.

For patients who want to keep using, the authors suggest doctors advise these patients to limit use as much as possible and to counsel patients who are at risk for heart problems that, while there is limited research, they may want to keep the potential risks of marijuana use in mind, even as the drug becomes more popular with legalization.

“This is something I’ve talked to patients about for a long time,” said Dr. Karol Watson, a cardiology professor at the David Geffen School of Medicine at UCLA who was not affiliated with the study. “I do a lot of research on pollution and what we see with that is that even at a low level, breathing in anything but air puts you at a risk for problems.”

Watson said she gets a lot of questions from patients about weed. When they do ask about it, she said she tells them that, “we don’t know.”

“There are some reports of heart attacks after smoking, but no systematic study of a real good magnitude, but I tell them I can’t endorse it. I don’t think it is good, but we just don’t know how bad it is,” she said.

Watson said she would support a study looking at heart attack rates five years before legalization and five years after in a state like Colorado, where recreational marijuana use has been legal since 2012, to see if the rates went up.

Unfortunately, she said, there just isn’t enough research to know for sure, “but I do think it’s never a good idea to suck something deep into your lungs, other than air.”