More than 2 million Americans with heart disease report using or having used marijuana in the past, prompting new recommendations that physicians routinely screen their patients for use of the drug.

The effects of marijuana on heart health remain poorly understood, in part due to the plant’s classification as a Schedule 1 drug by the US Drug Enforcement Agency – the government’s strictest schedule which defines a drug as having “no currently accepted medical use and a high potential for abuse”. What researchers do know, they largely gather through real-world data, such as the National Health and Nutrition Examination Survey.

Researchers at Brigham and Women’s Hospital turned to data gathered in 2016 to determine potential health risks associated with the rise in marijuana use in the US and compared it against existing scientific literature on marijuana use. Writing in the Journal of the American College of Cardiology, they report that the findings were “eye-opening”.

Previous studies suggest a link between marijuana use and heart attack and stroke, leading researchers to believe using marijuana may be a potential trigger for a cardiovascular episode. In one small study, marijuana use was shown to bring on angina, or chest pain, more quickly in patients with coronary heart disease when compared with smoking a placebo. In addition, marijuana may induce changes in the inner lining of blood vessels or alter blood flow as well as increase cellular stress and inflammation (though cannabinoids have also shown to be a potent anti-inflammatory).

"In addition to the 2 million marijuana users with diagnosed cardiovascular disease, many more may be at risk," said study author Ersilia DeFilippis, of Columbia University Irving Medical Center, in a statement. "With many adolescents and young adults turning to marijuana, it is important to understand the cardiovascular implications they may face years down the line."

As potency increases, the researchers note that this may impact how marijuana and its compounds affect the human system. For example, cannabinoids inhibit certain enzymes in the body which can affect how certain drugs are metabolized, particularly those used to treat heart disease.

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

To combat this, the researchers advise physicians to make screening for marijuana a regular part of their examination process.

"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 said.

"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."

One in seven American adults reported using marijuana in 2017, Reuters noted at the time. That number goes up in the 11 states where marijuana is legal both for recreational and medicinal purposes for adults over the age of 21. An additional 33 states allow marijuana for medicinal purposes, reports Business Insider. Research suggests that marijuana and its more than 100 compounds may have health benefits, from treating chronic pain and muscle spasms to reducing seizures in children with severe epilepsy.