As more states legalize marijuana, heart specialists are warning that “natural” does not always mean harmless. One cardiac surgeon, Dr. Jeremy London, has stepped forward to explain what regular marijuana smoking can do to the body—particularly the heart and lungs.

Doctors recently highlighted a troubling condition called cannabis hyperemesis syndrome, or CHS. People who develop CHS can suffer intense stomach pain and repeated vomiting that may last for days. Although still not fully understood, researchers believe it may result from long-term overstimulation of the body’s endocannabinoid system.

Dr. London stresses that the lungs were designed for one purpose—moving air in and out. Introducing smoke of any kind, including marijuana, can be damaging. He says many users assume it’s safe because it comes from a plant, but the evidence tells a different story.

According to Dr. London, people who regularly smoke marijuana face a 1.3 to two-times higher risk of coronary artery disease and heart attack compared with non-users. This increased risk is similar to what doctors see with high blood pressure and elevated cholesterol.

He also described what he has witnessed in the operating room. Some longtime marijuana smokers—who have never touched cigarettes—have lungs so darkened and blocked that they resemble those of coal miners. Several have needed bypass surgery due to severe arterial damage.

Other medical organizations echo these warnings. Research has linked cannabis use to a higher chance of heart attacks, and the American Lung Association advises against smoking marijuana because of its potential to harm lung tissue.