More than 2 million Americans with heart conditions report that they have used marijuana, but many questions remain about the drug’s effects on the heart, according to a review published Monday in the Journal of the American College of Cardiology.

What is known, however, is that the drug can interact with common heart medications, including statins and blood thinners, potentially putting patients at risk, the review said.

Using marijuana while on a statin or a blood thinner can change how these drugs work in the body, said lead author Dr. Muthiah Vaduganathan, a cardiologist at the Brigham and Women’s Hospital in Boston.

That’s because the same liver enzymes that break down statins or blood thinners also break down the compounds in marijuana, he said. Using them simultaneously with marijuana can change the effectiveness or the potency of the medications.

For example, marijuana can increase the levels of the blood thinner warfarin in the body, which can lead to excessive bleeding, according to the review. One case report published last year found that people using marijuana while on warfarin may need to reduce their dosage by as much as 30 percent. With statins, pot can boost the potency, which could lead to dangerous drops in blood pressure.

If people who are on heart-related medications do choose to use marijuana, it’s important that they tell their doctor and their pharmacist, so the medication dosages can be adjusted if needed.

“The first step is having an open discussion with clinicians, because it does influence some parts of their care,” Vaduganathan said. And certain heart health patients should avoid pot entirely.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Based on the available evidence, the highest risk patients — such as people who recently had a heart attack or were hospitalized for a heart problem — should be counseled against using marijuana, or advised to limit its use, he said.

People not taking heart medications are also advised to pay attention to how pot affects the heartbeat.

“Always pay close attention to the way marijuana affects the heartbeat.” said Dr. Sergio Fazio, a professor of cardiovascular medicine at Oregon Health & Science University in Portland. Palpitations —when the heart races, pounds or skips a beat — are a common side effect of marijuana and could be dangerous for someone with an existing condition.

“If you sometimes feel your heart pounding or beating out of whack, these are signals that you should not ingest marijuana,” Fazio, who was not involved with the new review, said.

Dr. Carl Lavie, medical director of cardiac rehabilitation and preventive cardiology at the Ochsner Clinic in New Orleans, said that THC, the main psychoactive ingredient in cannabis, may cause platelets in the blood to clump and form clots, increasing a person’s risk for stroke or heart attack.

Smoke, vape or eat?

How users ingest marijuana also plays a role in how risky the drug may be for people with heart problems.

The latest data from the Centers for Disease Control and Prevention’s Behavioral Risk Factor Surveillance System, which surveys people’s health activities, found that nearly 80 percent of adult marijuana users reported smoking the drug.

“Inhalation and vaping of anything that isn’t meant to be in the lungs, even water, is a bad idea,” Fazio said.

Though marijuana affects the entire cardiovascular system regardless of how it’s ingested, Fazio said that edibles are the safest route, as smoking poses many of the same risks as inhaling tobacco. And vaping, as demonstrated by the ongoing outbreak of serious illnesses, introduces a bevy of risks.

Still, edibles also come with risk. It’s very easy to ingest a much higher dose of marijuana when consuming it as an edible. People who opt for edibles should understand exactly how much is in each serving and should not take more if they don’t feel its effects right away.

“The problem is knowing the purity and dose, and this is extremely variable among different products regardless of the delivery system,” Lavie told NBC News in an email. “A low dose of pure THC is safer than a high dose of THC that has many bad contaminants.”

“I’m not sure people are always aware of what exactly they are getting,” he added.

Synthetic forms of marijuana, including K2, can also pose severe health problems, Lavie said. In 2018, contaminated synthetic pot in Illinois caused bleeding disorders that were so severe some doctors nearly mistook patients’ symptoms for Ebola.

Synthetic pot can also be up to 100 times as potent as the THC in the cannabis plant, making these versions even more dangerous for people taking statins or blood thinners.

It’s possible that marijuana isn’t always bad for the heart, however. Using medical marijuana as prescribed could lead to less stress, for example.

“Anytime someone says that they were able to get eight hours of peaceful sleep because they used a little bit of marijuana, their cardiovascular health will likely be better off with the use of marijuana,” Fazio said. “When you move to the purely recreational use, that’s where the risks associated with heart problems potentially outweigh the benefits.”

Follow NBC HEALTH on Twitter & Facebook.