It’s been known for many years that people who smoke tobacco have poor blood vessel function. Now, a team of researchers at UC San Francisco has shown that people who smoke marijuana have the same problem.
The study, presented on November 13, 2023 at the American Heart Association annual meeting in Philadelphia, showed that marijuana smokers had poor vascular function compared to non-users.
“Even though many people believe that cannabis smoke is harmless, actually, smoke from burning plant material contains thousands of chemicals, and cannabis and tobacco smoke are chemically and physically similar,” said lead author Leila Mohammadi, MD, PhD, a senior research scientist in the UCSF division of Cardiology.
In this study, called CANDIDE (CANnabis: Does It Damage Endothelium), the researchers recruited people who smoke marijuana at least three times per week, people who smoke at least five tobacco cigarettes per day, people who avoid all kinds of smoke but take at least three THC edibles per week and people who aren’t exposed to any smoke or THC at all.
They found that the tobacco smokers had poor vascular function relative to the non-users (the arteries were less able to open up to increase blood flow when necessary), confirming what was already known from prior research. However, the marijuana smokers had the same problem.
In a surprise finding, participants who consumed THC edibles also had poor vascular function, even though previous studies in rats had shown that THC is not required for marijuana smoke to impair vascular function, and vascular function is also impaired by tobacco smoke, which doesn’t contain THC.
“Wish we had better news about THC”
To better understand why marijuana smoke without THC and THC without marijuana smoke both appear to impair vascular function, the team exposed endothelial cells (the cells that line the inside of blood vessels and control their function) to blood samples from the different study participants. It’s already known that the blood of tobacco smokers lowers the production of nitric oxide by endothelial cells, a molecule that is crucial for vascular function. The blood samples from marijuana smokers also made the cells produce less nitric oxide than cells exposed to blood from non-users, but the blood from the THC edible users did not affect nitric oxide production.
“Our findings suggest that the impairment caused by smoking cannabis versus edible THC use occurs by different processes in the body,” said Mohammadi.
“I wish we had better news about THC,” said senior author Matthew Springer, PhD, UCSF professor of Medicine, in the division of Cardiology. “I typically emphasize that marijuana smoke is still smoke and inhaling it carries some of the same cardiovascular risks of tobacco smoke, so, if you want to take THC, it’s best to do it in a way that doesn’t involve inhaling smoke. However, now it seems THC is also interfering with blood vessel function for different reasons. It’s interesting from a scientific standpoint, but boy does it complicate the public health messaging.”
Prior to the CANDIDE study, Springer and his UCSF research group had done many studies in rats, which have similar cardiovascular responses to tobacco smoke as humans do, and which can be studied under tightly controlled experimental conditions. Over the past decade, they have reported that even just one minute of exposure to secondhand smoke from marijuana reduces blood vessel function in rats similarly to the effects of secondhand smoke from tobacco, and rats exposed to smoke from marijuana or tobacco once a day for eight weeks have similar harmful effects in their hearts.
These studies have raised public awareness of the similar harmful effects of tobacco and marijuana smoke on the heart and blood vessels, including exposure to secondhand smoke.
“In the past, our findings were based on rats, not humans,” said Springer. “Now we can finally confirm that they have indeed been relevant to humans, as we see that human marijuana smokers and tobacco smokers have similarly unhealthy arteries.”
Co-authors: Mina Navabzadeh, PharmD; Daniel Han, BA; Emma Reagan, BS, BA; Jordan Naughton, BA, Lylybell Zhou, BS; Rahul Almeida, Kathryn Park, Keith Uyemura, Natasha Goyal, MBBS; Poonam Rao, MBBS; Judith Hellman, MD; Jing Cheng, MD, PhD; and Gregory Marcus, MD.
Funding: California Department of Cannabis Control, National Institute on Drug Abuse, Elfenworks Foundation and Roy E. Thomas Medical Foundation.