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But it took the rats three times longer to recover from a minute of secondhand marijuana smoke, compared with tobacco smoke, the researchers found.
"If people think marijuana smoke is somehow exempt from the harmful effects of tobacco and many other kinds of smoke, this is evidence that it is not," said study senior author Matthew Springer, a professor of medicine in the division of cardiology at the University of California, San Francisco.
The rats' arteries carried blood less efficiently for at least 90 minutes following exposure to marijuana smoke, but began recovering from tobacco smoke within 30 minutes, the study found.
Smoke from both sources impaired the blood vessels' ability to expand and improve blood flow by more than half, an effect that increases short- and long-term risk of a stroke or heart attack, Springer explained.
"While the effect is temporary for both cigarette and marijuana smoke, these temporary problems can turn into long-term problems if exposures occur often enough," Springer said. "It may increase the chances of developing hardened and clogged arteries."
Smoke from any burning source contains large amounts of harmful chemicals, even if it's from a naturally grown plant like tobacco or marijuana, said Dr. David Friedman, chief of heart failure services at Northwell Health Long Island Jewish Valley Stream Hospital in Valley Stream, N.Y.
"This study kind of confirms what we already know -- smoke is toxic," Friedman said. "That's why firefighters wear all kinds of protective gear when they go into a burning building."
Repeated exposure could do long-term damage to blood vessels, potentially causing a cascade of inflammation that causes arteries to harden, he said.
Officials with NORML, a pro-marijuana legalization group, said these lab results don't jibe with previous studies that found no link between marijuana smoke and increased risk of heart disease, hardened arteries, emphysema, airway cancers or other tobacco-related health problems.
"Although the acute effects are certainly intriguing in these rodents, I would not generalize to humans given the data on cannabis use and arterial, cardiac or pulmonary problems," said Mitch Earleywine, a member of NORML's advisory board and a professor of psychology at the State University of New York at Albany. "Most heavy users do not suffer from medical conditions as long as they don't smoke tobacco."
Springer agreed that the new study doesn't directly prove harm in humans, but noted it has been established that rat circulatory systems have enough similarities with those of humans to make them useful in the laboratory.
"There's a logical flow here that I hope everyone understands," he said. "The blood vessel function that we're looking at is very similar in humans and rats. We know that humans and rats respond very similarly to tobacco smoke, including secondhand smoke. It's a reasonable extension to think that humans will respond to marijuana smoke similarly to the rats."
Based on these findings, marijuana users should avoid smoking around others, Springer said. In states where marijuana is legal, clean air laws should forbid public pot smoking in exactly the same ways they forbid tobacco smoking.
"We just need a shift in the thinking, where we think about marijuana smoke the same way we think about cigarette smoke," he said.
To protect their own health, pot users should consider switching to edible marijuana products, Springer said. Using an e-cigarette leaf vaporizer also could be a safer alternative, but that needs to be tested, he added.
Earleywine agreed there are options available for people concerned about smoke.
"My own work on the vaporizer suggests that those who are concerned about secondhand cannabis smoke can encourage their pals to turn to vaporized or edible products," he said.
The findings were published July 27 in the Journal of the American Heart Association.
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