By Sandee LaMotte, CNN

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Healthy people who regularly smoked marijuana or consumed THC-laced edibles showed signs of early cardiovascular disease similar to tobacco smokers, a new small study found.

“To my knowledge, it’s the first study looking at THC’s impact on vascular function in humans,” said senior study author Matthew Springer, professor of medicine at the University of California, San Francisco.

“We’re looking at a window in the future, showing the early changes that may explain why smoking marijuana has been linked to later heart disease,” Springer said. “It appears the act of smoking and the THC itself both contribute to those changes in different ways.”

Tetrahydrocannabinol, or THC, is the component of marijuana that provides a high. Prior research on mice found damage to blood vessels that supply oxygen to vital organs after exposure to marijuana smoke, Springer said. Whether marijuana smoke would impact the human vascular system, however, was unknown.

“We found that vascular function was reduced by 42% in marijuana smokers and by 56% in THC-edible users compared to nonusers,” lead study author Dr. Leila Mohammadi, an assistant researcher in cardiology at the University of California, San Francisco, said in an email.

The research only shows an association, Springer said. “We can only state that the cannabis users have poor vascular function, not that cannabis use causes poor vascular function,” he said via email.

The findings on THC-laced edibles was surprising, said Dr. Andrew Freeman, director of cardiovascular prevention and wellness at National Jewish Health in Denver.

“Could it be that other forms of marijuana — teas, tinctures, edibles — are perhaps not as benign as we once thought?” said Freeman, who was not involved in the study. “We need larger studies to make a better conclusion about this finding.”

Risk of damaged blood vessels

A single layer of endothelial cells lines all of the body’s blood vessels. When functioning properly, these specialized cells release chemicals such as nitric oxide that control the relaxing and contracting of the canal, thus regulating blood flow. Healthy endothelium cells also play a role in local cell growth and help prevent blood clotting.

When endothelial cells are inflamed, plaque buildup can increase in arteries over time, potentially causing heart attacks, strokes and heart failure. Damage to small blood vessels can also cause kidney and lung disease, comas, delirium, and dementia.

The study, however, did not measure plaque, so the findings do not mean that blood vessels were currently blocked, Springer said.

“The vessels just don’t grow in diameter in real time when they need to pass more blood, indicating an unhealthy vessel wall that presages later cardiovascular disease,” he said.

Prior studies have found strong links between marijuana use and later cardiovascular disease. A February 2024 study found smoking, vaping or eating marijuana led to a significantly higher risk of heart attack and stroke, even if a person had no existing heart conditions and did not smoke or vape tobacco.

Stroke risk rose 42% and the risk of heart attack rose 25% if cannabis was used daily, and risk climbed as the number of days of use of marijuana rose, the study found.

Using marijuana every day can raise a person’s risk of coronary artery disease by one-third compared with those who never partake, a February 2023 study found.

The American Heart Association advises people to refrain from smoking or vaping any substance, including cannabis products, because of the potential harm to the heart, lungs and blood vessels. Guidance released in 2020 pointed to studies that found heart rhythm abnormalities, such as tachycardia and atrial fibrillation, could occur within an hour after weed containing THC was smoked.

Small pilot study

The 55 participants in the study were divided into three groups: people who smoked (not vaped) marijuana three or more times a week for at least a year, people who consumed THC edibles at least three times a week for at least a year, and nonusers. None of the 18- to 50-year-old people in the study were tobacco smokers or vapers, and all had little exposure to secondhand tobacco smoke.

Researchers performed an ultrasound on the major artery in the upper right arm in each person, then applied an extremely tight blood pressure cuff for five minutes. After the cuff was removed, the artery was rescanned to see how well it had dilated, or widened, to handle the increased flow of blood, a process that needs the release of nitric oxide to occur.

Damage to the endothelial cells that regulate dilation was related to the dose, according to the study. Study participants who used more marijuana had a greater risk of damage to blood vessels that carry oxygen to the body’s organs.

“Higher cannabis use — whether smoked or ingested — is associated with poorer vascular function, highlighting the cardiovascular risks that increase with higher potency and frequency of use,” Mohammadi said.

Additional tests of the neck and thigh checked the stiffness of each person’s blood pressure walls. Compared with people who never used marijuana, cannabis users didn’t appear to have additional stiffness of the walls of blood vessels, the study found.

The roles of smoking and THC

A separate analysis added blood serum from cannabis smokers and edible users to commercially purchased cultures of endothelial cell in the laboratory. Blood from people who used edibles laced with THC didn’t appear to harm the cells — they continued to produce adequate nitric oxide. However, the endothelial cells incubated in serum from marijuana smokers released 27% less nitric oxide than those treated with blood from nonsmokers.

That evidence is similar to what is found in tobacco, Springer said, pointing to a prior study by his team showing endothelial cells incubated in serum from tobacco smokers released 39% less nitric oxide than nonsmokers.

“The observations that marijuana smokers and THC users each have poor vascular function might make people conclude that the THC is responsible for all of this and the smoke is irrelevant,” Springer said.

That would be an ill-advised conclusion, according to Springer, as there are reasons to believe that marijuana smoke itself is as responsible as THC for the damage to blood vessels.

“Rats exposed to marijuana smoke with no cannabinoids at all also had vascular and cardiac impairment, plus tobacco smoke is known to cause heart disease and it has no THC,” Springer said.

“So you do yourself no favors by switching from smoking tobacco to marijuana. Smoking marijuana just gives you a double hit — the smoke and the THC,” he said.

As for marijuana edibles, teas, tinctures and the like?

“In people, there’s like a Goldilocks zone for everything — too much doesn’t do right, too little doesn’t do right, but just right does fine,” Freeman said. “We need further investigations to see if there is a Goldilocks zone to be found.”

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