FRIDAY, Dec. 20, 2019 (HealthDay News) -- Regular pot use might potentially cause changes in the heart's structure, a new study suggests.
People who regularly use marijuana tend to have a larger left ventricle, which is the main pumping chamber of the heart, according to the findings.
Routine stoners also appeared to have early signs of impaired heart function, measured by how the fibers of the heart muscle deform during contraction, the British researchers said.
But the heart appears to recover in both size and function once a person stops using marijuana, the study found.
"Those who used cannabis regularly had enlargement of the heart, but when they saw the patient had stopped using the drug, it seems the function was improved," said Dr. Mary Ann McLaughlin, a cardiologist at Mount Sinai Hospital in New York City who reviewed the findings.
The researchers warn that the study "should be interpreted with caution, and more research is required to understand the potential mechanisms and dose-related effects of cannabis use.
But the results jibe with concerns people have had about the effects of pot on the heart, said Dr. Martha Gulati, editor-in-chief of the American College of Cardiology's patient education website, CardioSmart.org.
"There's so much we don't know about cannabis use and its effect on the heart, but one of the things we do know is that when people use marijuana, particularly if they smoke it, the heart rate and the blood pressure go up, and the heart has to work harder," Gulati said.
The study, led by Dr. Mohammed Khanji, from Queen Mary University of London, involved more than 3,400 people in the United Kingdom, of whom 47 were current regular users of marijuana and 105 were previously regular users. The rest rarely used pot, if at all.
Khanji and his team studied MRIs to assess both the structure and function of participants' hearts.
Although the size of the left ventricle was larger in regular pot users, the other three chambers of the heart remained unaffected, researchers added. The heart also appeared to pump the same amount of blood, regardless of marijuana use.
The findings were published Dec. 18 in JACC Cardiovascular Imaging.
McLaughlin characterized the study as "hypothesis-generating," rather than a solid result.
"It's not a definitive answer by any means, but it gives us reason to investigate the effect of marijuana on heart structure," she said.
Because the study was observational, it's hard to know whether other factors might have caused the heart structure changes associated with marijuana use, McLaughlin said.
"Alcohol can also cause similar types of changes in the left ventricle with chronic drinking, which can get better when people stop drinking," she said. "They said they adjusted for alcohol use in this study, but the question is whether the use was adequately assessed."
Europeans also are known to mix their marijuana with tobacco, which has notoriously harmful effects on the heart, said Paul Armentano, deputy director of NORML, a group advocating reform of U.S. marijuana laws.
Experts also noted that the number of pot users in this study was very small.
"Replications with more extensive measures and a dramatically larger sample of cannabis users do seem quite justified," said Mitch Earleywine, a professor of psychology at the University at Albany-State University of New York and a NORML advisory board member.
If marijuana is indeed the cause, it's hard to say what exactly about pot is affecting the heart, physicians say.
It could be the smoke that's being inhaled that places strain on the heart, or it could be THC, the chemical in pot that gets you high, said Dr. Larry Allen, a professor of cardiology at the University of Colorado School of Medicine in Aurora.
THC has been shown to increase heart rate and blood pressure, cause blood to clot more easily, and affect the inner lining of blood vessels, he said.
"We have some basic laboratory data that suggests there may be adverse health effects of THC," Allen said.
Until there are more answers, people with heart problems might want to avoid pot, the experts said.
"If you have heart disease, you should really use marijuana with caution," Gulati said. "In fact, I, as a cardiologist, would recommend you not to use it because of the physiologic effects of increasing your heart rate and putting more demand and stress on the heart."