Young people who use marijuana may be at risk for heart attacks and other serious cardiovascular problems, a new study suggests.

Researchers reviewed records from the French Addictovigilance Network, a national system of centers in France that gather information about drug abuse and dependence. From 2006 to 2010, they found 35 reports of patients who had experienced cardiovascular complications following cannabis use. The study was published Tuesday in the Journal of the American Heart Association.

About 85% of the patients were men and their average age was 34. In nine cases the patient died.

While the study doesn't identify a particular level of risk of marijuana use, it does call for more research on the subject, and warns doctors that there seems to be at least some danger.

"Currently people think (cannabis is) harmless," says lead study author Emilie Jouanjus, at the Université Toulouse III – Paul Sabatier. "What I think is very important from my work is that we see cannabis use may lead to very serious complications on the cardiovascular system."

Jouanjus was surprised to find all but one of the patients with cardiovascular problems were under the age of 50. She believes this study shows only the tip of the iceberg, as people usually under-report their cannabis use.

"This study shows a some preliminary evidence of cardiovascular harm from marijuana but isn't conclusive,” Dr. Allen Taylor, chief of cardiology at Georgetown University School of Medicine, told CNN in an e-mail. “The study's limitations are important in that we can't know how high the risk is, just that there is a signal of risk between marijuana smoking and heart troubles.”

Taylor says it’s a shame that more isn't known about the potential harms of cannabis.

"It seems that public perception is ahead of the science," he says. "We should remain open to the scientific facts as they evolve."

Only 20% of patients in the study had a known history of heart disease or risk factors for heart disease. The other patients may have had unknown cardiovascular risk factors or a history that was not documented in the medical file.

“I'm not saying that any user of cannabis would suffer from any of these complications," Jouanjus says. But "we do not have enough information to say that cannabis use is safe."

Dr. Shereif Rezkalla. who wrote an editorial accompanying the study in JAHA, said doctors should be required to ask about marijuana use when they see a cardiac patient.

“The perception that marijuana is safe is deep-seated in the public and even amongst some health professionals,” says Rezkalla, a cardiologist at Marshfield Clinic in Marshfield, Wisconsin.

Taylor cautions that while this study doesn’t provide a final answer on the link between cannabis and serious heart problems, “it at least gives one pause to suggest to patients it may not be a heart-healthy habit."

Previous research has suggested marijuana use may double the risk of stroke in young adults.