Early in his career operating on lung cancer patients, Dr. Raja Flores knew most were cigarette smokers. But through the years, Flores, a thoracic surgeon at Mount Sinai Medical Center in New York, noticed a startling pattern: Some of his patients had never smoked a tobacco cigarette. They smoked a different drug: marijuana. And they had developed a much more aggressive form of lung cancer.

Initially, Flores didn’t consider there could be a connection between marijuana and lung cancer. The research linking pot smoking with cancer was scant and largely inconclusive. But as the numbers grew, Flores wondered if he was seeing some kind of grim new trend.

“I said to myself, ‘wait a minute, here’s another person in his 40s who never touched a cigarette and the cancer is all over the place,” Flores told NBC News. “It’s so bad I can’t even operate.’”

Flores first raised his concerns about harms from smoking marijuana in SurvivorNet, an online community for cancer patients and experts. He acknowledges there isn’t scientific proof that smoking cannabis causes lung cancer. But he worries that the combination of widespread legalization and marketing of marijuana’s potential health benefits are contributing to the belief that cannabis is an entirely benign drug. In fact, a nationally representative survey of U.S. adults published last summer found that nearly a third thought that smoking or vaping weed could protect a person’s health.

Many Americans held a similar view of tobacco before evidence accumulated showing that cigarette smoking could cause lung cancer, said pulmonologist Dr. Panagis Galiatsatos, a critical care instructor at Johns Hopkins School of Medicine and director of the tobacco treatment clinic at Johns Hopkins Medicine.

Until the number of cigarette-related lung cancer cases accumulated to a level that no one could ignore, there were even respected scientists who dismissed warnings about the potential dangers associated with smoking tobacco, Galiatsatos said.

Does marijuana use cause lung cancer?

Most research says no, but that doesn’t mean that smoking joints long-term comes without consequences.

A 2016 review monitoring health concerns related to marijuana use found strong evidence for an association between daily or near-daily marijuana use and chronic bronchitis, an inflammation of the airways of the lung that causes cough, wheezing and shortness of breath.

“There are respiratory effects to using smoked marijuana for long periods of time,” said Dr. Russell Bowler, director of the COPD clinic at National Jewish Health in Denver. He's a member of the the Retail Marijuana Public Health Advisory Committee for the Colorado Department of Public Health and Environment, which commissioned the study.

Daily or near-daily marijuana use may be associated with bullous lung disease, a condition where air pockets are formed within the lung disrupting its function, and pneumothorax, a condition that commonly leads to collapsed lungs in young people, Bowler's research found.

Bowler said there needs to be more research into smoking marijuana and the development of lung cancer.

“There’s very little published [data] on secondhand exposure and insufficient data to make conclusions about its health effects,” Bowler told NBC News.

One reason for the lack of data: Studies on marijuana cannot be performed in the U.S. because, on the federal level, the drug remains in the same class as heroin and LSD, a Schedule I drug. That means marijuana is designated as having “a high potential for abuse” and “no currently accepted medical use.”

However, public health advocates of U.S. cannabis research believe marijuana laws should have no bearing on a scientist’s ability to study the drug’s consequences on a population.

Don't smoke anything

Researchers comparing the smoke of cannabis and tobacco say they may be similar in how they damage the lungs.

“The smoke of marijuana contains many of the same volatile chemicals found in tobacco smoke that are injurious to lung tissue,” pulmonologist Dr. Donald Tashkin, emeritus professor of medicine at the David Geffen School of Medicine, UCLA, told NBC News.

“As a pulmonologist, I advise all my patients not to smoke anything,” Tashkin said.

On the surface, pot smokers would seem to be at a lower risk than cigarette smokers because they light up fewer times a day. But researchers have found that far more carcinogens, like tar, end up in the lungs when a joint is smoked than a cigarette.

This occurs because marijuana is smoked differently than a normal cigarette. Traditional vaping or joint smoking usually involves inhaling the marijuana smoke deeply into the lungs and holding it in, which gives the toxins more contact with the lungs. Also, many marijuana users smoke a joint all the way to the end.

Tar, the sticky stuff left after burning, is concentrated at the end of a joint and contains high levels of harmful substances that can injure lung cells.

Some research has shown that smoking one joint is comparable to smoking anywhere from four to 20 cigarettes. These findings are one reason why Tashkin expected to find a link between smoking pot and developing lung cancer.

“Regular smoking of marijuana by itself causes visible and microscopic injury to the large airways that is consistently associated with an increased likelihood of symptoms of chronic bronchitis that subside after cessation of use,” Tashkin wrote in a 2013 study on the effects of habitual marijuana use on the lungs.

He also found that pre-cancerous alterations developed in the lungs, but it often did not lead to cancer.

“In human studies of long-term marijuana users, we found widespread pre-cancerous changes on the lungs,” Tashkin said. “That doesn’t mean that if you have these changes you will develop lung cancer. But if it’s there you are at an increased risk of doing so.”

Tashkin, who has co-authored numerous studies on the impact of marijuana on the lungs over the past 30 years, has even re-analyzed his own data. He only found an increased risk of lung cancer in heavy smokers in a small number of patients. The number was so small that Tashkin didn’t believe it could be used as firm evidence to support the conclusion that pot could cause cancer.

Marijuana more potent than ever

In the U.S. marijuana is legal by a doctor’s order in 33 states and recreationally in 11 states plus the District of Columbia.

Marijuana, unlike tobacco, does not have any controls on its strength or quality so people don’t use the same amount in one dose. Also, those who use medical marijuana for non-specific conditions such as pain treatment, may vary in the amount of drug they need to provide relief. These factors make it hard for researchers to set standards to measure the drug’s effects.

More potent forms of marijuana, sometimes called skunk marijuana, may be one reason people don't develop lung cancer, despite ingesting the same cancer-causing agents as combustible cigarettes, Browler explains.

“Now in Colorado people get a product that routinely has 30 percent THC. That means people are smoking a more potent product and may be smoking fewer times per day to receive the same effect,” said Browler. “Some cigarette smokers smoke 1-3 packs a day, that’s up to 60 cigarettes. You wouldn’t be alive if you were smoking 60 joints a day at 30 times the amount of THC.”

Smoking may still be the most popular form of using marijuana, but using edibles has grown in popularity and that doesn’t affect the lungs, he added.

One thing experts agree on is that more research is needed.

In the early 50s, some prominent scientists, questioned the idea that smoking could cause lung cancer, but it wasn’t until there were more than 30 million cigarette smokers in the U.S. that the danger to the lungs started to become clear, Galiatsatos said. [An earlier version of this story incorrectly attributed this statement to Flores.]

“If we turn our back on this now, we may have a big disaster later on,” Flores said.