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Millions of Americans use marijuana for medical reasons, such as reducing anxiety or helping ease the side effects of cancer therapy. Millions more turn to it for recreational reasons, saying it helps them relax. Now a new study says those who smoke marijuana regularly might be putting themselves at increased risk of heart disease and heart attack.

There has long been debate about the benefits versus the risks for using cannabis, with evidence on both sides to support each position. For example some studies have shown taking oral cannabinoids can help people cope with the nausea brought on by chemotherapy. Other studies have shown that regular use of marijuana can cause problems such as marijuana use disorder, a condition where the user is showing physical or psychological problems but has difficulty controlling or reducing their use of cannabis.

Now this latest study, from researchers at Stanford Medicine,  shows that THC, the psychoactive part of the drug, can cause inflammation in endothelial cells. These are the cells that line the interior of blood vessels. When these cells become inflamed it can cause a constriction of the vessels and reduce blood flow. Over time this can create conditions that increase the risk of heart disease and heart attack.

The researchers, led by Dr. Joe Wu, began by analyzing data from the UK Biobank. This included information about some 35,000 people who reported smoking marijuana. Of these around 11,000 smoked more than once a month. The researchers found that regular marijuana smokers:

• Were significantly more likely than others to have a heart attack.
• Were also more likely to have their first heart attack before the age of 50, increasing their risk of subsequent attacks.

The team then used the iPSC method to create human endothelial cells and, in the lab, found that THC appeared to promote inflammation in the cells. They also found signs it created early indications of atherosclerosis, where there is a buildup of fat and plaque in the arteries.

They then tested mice which had been bred to have high levels of cholesterol and who were given a high fat diet. Some of the mice were then injected with THC, at a level comparable to smoking one marijuana cigarette a day. Those mice had far larger amounts of atherosclerosis plaque in their arteries compared to the mice who didn’t get the THC.

In a news release, Dr.Wu, the lead author of the study, said: “There’s a growing public perception that marijuana is harmless or even beneficial. Marijuana clearly has important medicinal uses, but recreational users should think carefully about excessive use.”

On the bright side, the team also reported that the damage caused by THC can be stopped by genistein, a naturally occurring compound found in soy and fava beans. The study, in the journal Cell, also found that genistein blocked the bad impact of THC without impeding the good impacts.

“As more states legalize the recreational use of marijuana, users need to be aware that it could have cardiovascular side effects,” said Dr. Wu. “But genistein works quite well to mitigate marijuana-induced damage of the endothelial vessels without blocking the effects marijuana has on the central nervous system, and it could be a way for medical marijuana users to protect themselves from a cardiovascular standpoint.”