Can Smoking Marijuana Cause Lung Cancer?

Marijuana, both for recreation and medical use, is becoming legal in more states. Even as more people use it, health experts aren’t sure whether smoking marijuana raises your odds of getting lung cancer. Here’s what researchers know — and don’t know — about the connection.

“There hasn’t been nearly as much research on marijuana as on tobacco as a cause of cancer because of it being largely illegal,” says Otis W. Brawley, MD, the chief medical officer of the American Cancer Society in Atlanta.

Outside of issues with legality, studies have trouble controlling for marijuana use alone. Since many people who smoke pot also smoke tobacco, says Dr. Brawley, determining direct causation between pot and lung cancer is difficult.

Here are three things to consider when it comes to marijuana use and lung cancer.

Any kind of smoke can damage your lungs

Whether it’s tobacco smoke or marijuana smoke, your lungs still suffer, says Robert Schwartz, PhD, senior scientist at the Centre for Addiction and Mental Health in Toronto. “If you’re inhaling smoke, it doesn’t matter the kind, you’re creating damage,” he says. “My motto is you need to take the smoke out of dope.”

According to the American Cancer Society, tobacco contains at least 70 cancer-causing chemicals. About 30 of those 70 chemicals are also present in marijuana, says Dr. Brawley. Studies have found both positive and negative correlations between smoking pot and increased lung cancer risk, but inhaling smoke of any kind harms the lungs.

What about other forms of marijuana consumption, like edibles and vaping? Thorough research has yet to be done on these methods, says Schwarz. Since consuming weed in vapor form still releases particles into the lungs, this method could be a plausible lung-cancer contributor.

Why It Might Be Harmful

The link between tobacco smoke and lung cancer is well-known. Studies show that marijuana smoke has many of the same harmful substances as tobacco, and often more of them. Among the hazards are:

  • Benzo(a)pyrene
  • Benz(a)anthracene
  • Phenols
  • Vinyl chlorides
  • Nitrosamines
  • Reactive oxygen species

People also smoke marijuana in a different way than tobacco, possibly posing greater danger to the lungs:

  • You usually inhale marijuana smoke deeply and hold it in, which gives the toxins more contact with your lung tissue and more chance to stick there.
  • You generally a smoke a joint all the way to the end. Tar, the sticky stuff left after burning, has high levels of harmful substances, and it’s concentrated at the end of a joint.

When scientists looked at lung tissue of some people who smoked weed regularly, they found changes that are known to signal the future growth of cancer.

There’s a lack of quality control

When marijuana users light up, there’s a chance they’re not only smoking pot, says Schwartz. “In Canada, 30% of people who smoke marijuana mix tobacco in it,” he says. Mixing drugs not only increases carcinogens, but also makes smoking in moderation difficult due to the addictive nature of tobacco.

The CIHR is still determining if people are substituting their cigarette habits for pot, which would be a positive effect of the drug, but Schwarz says it’s unlikely in older populations. “I would find it unlikely that we’re going to get huge transfers of cigarette smokers to marijuana, but for young people, they might pick marijuana over cigarettes.”

Strains of marijuana can vary widely too, so the effects of one batch may be different than another. “There might be a chemical that is high in concentration in one stash and that could be great for pain and nausea in cancer patients, but then the next stash that someone buys, that chemical could be low,” explains Dr. Brawley.

What’s more, synthetic marijuana—referred to by a number of names including Spice, K-2, Moon Rocks, and more—relies on a mixture of manmade chemicals sprayed onto miscellaneous dried plants and herbs and often incites erratic behavior in users. Synthetic cannabinoids are unregulated and using them can be life-threatening in some cases, warns the National Institute on Drug Abuse.

Questions Remain

Given what scientists already know, why is it so hard to say how smoking pot affects your chances of getting lung cancer?

Studies that have looked for a direct link between the two have conflicting results — some found evidence that ties marijuana to lung cancer, while other data show little to no connection.

The topic is also tough to investigate. Scientists say a few factors limit how reliable the research is.

Most of the research on marijuana dates to when it was still widely illegal. It’s hard to gather information about behavior that’s against the law. Most studies have asked people to report how often they smoked marijuana, and researchers know that these kinds of surveys, called “self-reported,” aren’t as reliable as when they collect data in other ways. That’s because people don’t remember their behavior perfectly or might underestimate or conceal how often they do something that others think is wrong.

Illegal marijuana, unlike tobacco, doesn’t have any controls on its strength or quality. People don’t use the same amount in one “dose.” That makes it hard for researchers to set standards to measure its effects.Another problem is that many people who smoke marijuana also smoke tobacco, sometimes mixed in the same cigarette. So if they get lung cancer, it’s impossible to sort out what substance caused it.

Some marijuana smokers in the studies have been fairly young, which could skew the results. Cancers can take time to grow.

On the other hand, most people who use weed don’t smoke as much as a tobacco user, which could lower their odds for a problem.

Animal research suggests that some chemicals in marijuana work against tumor growth, which could explain why lung cancer isn’t showing up as often as scientists might expect in people who smoke it. The studies on this are in their early days, and researchers need to take a deeper look into this theory.

The Future

Now that marijuana is legal in more places, growers are making the product more standard and stronger. More people are smoking pot, too.

Any link between marijuana smoking and lung cancer isn’t clear now, but researchers have a chance to move beyond some of the problems that have made studies unclear in the past.