It's not just that it's so common now (1 in 5 people use cannabis). They want to warn you about heart attack risk.

Until recently, there hasn't been a lot of conclusive evidence about the drug's health impacts. But two new studies suggest it carries very serious heart health risks, including that people under 50 who use cannabis are more than six times as likely to have a heart attack, compared to nonusers.

"Asking about cannabis use should be part of clinicians' workup to understand patients' overall cardiovascular risk, similar to asking about smoking cigarettes," Ibrahim Kamel, MD, MHA, a clinical instructor at the Boston University School of Medicine and lead author of both studies, said in a news release. "At a policy level, a fair warning should be made so that the people who are consuming cannabis know that there are risks."

One study, published Tuesday in the journal JACC: Advances, included 4.6 million adults ages 49 and younger who, at the start of the study, were relatively healthy, meaning they didn't have any major health conditions. Among the 4.6 million people were 93,267 people whose health records showed they were cannabis users. The cannabis users tended to be about five years older than non-cannabis users.

Along with being more likely to have a heart attack during an average three-year follow-up period, cannabis users were less likely to survive the heart event. Compared to nonusers, the cannabis group was four times more likely to have a stroke and twice as likely to have heart failure.

A second, forthcoming analysis, which will be presented at the upcoming American College of Cardiology conference in Chicago, offers clarity amid the sometimes conflicting messages about cannabis's impact on heart health. The new meta-analysis pooled information from 12 previous studies and included 75 million people. 

The researchers found that current cannabis users were 1.5 times more likely to have a heart attack, compared to people who reported not currently using the drug.

Because the studies rely on past reported usage, there is some uncertainty about the findings, particularly since other things that aren't readily apparent may impact the results.

"We should have some caution in interpreting the findings in that cannabis consumption is usually associated with other substances such as cocaine or other illicit drugs that are not accounted for," Kamel said. "Patients should be forthcoming with their doctors and remember that we are their number one advocate and having the full story matters."

It's still unclear what happens in the body that links marijuana use to issues with the heart and blood vessels, but scientists think it could be related to heart rhythm changes, increased demand for oxygen by the heart muscle, or difficulty for blood vessels to relax and expand, thus interrupting blood flow. The researchers noted that one study in the meta-analysis found that heart attack risk peaked about one hour after using marijuana.