Is Marijuana Safe?

Medically Reviewed by Michael W. Smith, MD on February 25, 2014
From the WebMD Archives

Is the idea of marijuana as an illegal drug starting to go up in smoke?

Two states have already legalized pot for recreational use. And since polls show that most Americans are in favor of the practice, it might not be long before joints are rolled and bongs are smoked in many more states without fear of jail time.

Even though the laws are shifting, the debate over legalized marijuana continues. Although some say marijuana is just as safe as, or even safer than alcohol, others argue that pot causes a lot more harm than just a serious buzz.

Although there isn't much evidence that the occasional toke leads to long-term health problems, researchers are concerned that long-term, heavy pot smoking can leave lingering effects.

"I don't think we can definitively say it is safe," says Jeanette Marie Tetrault, MD, FACP. She's an assistant professor of medicine at the Yale University School of Medicine.

Ongoing marijuana use has been linked to lung changes, memory loss, and a number of other health problems.

Your Body on Marijuana

When you smoke pot, THC and other chemicals travel from your bloodstream to your brain. THC causes the feel-good "high."

Here are some of the other effects you'll feel:

  • Trouble thinking and remembering
  • Bloodshot eyes
  • Dry mouth (cotton mouth)
  • Increased appetite (the "munchies")
  • Fast heart rate
  • Slowed coordination

Marijuana Smoke: What's in It?

Marijuana smoke contains about 60 chemicals called cannabinoids. The best-known of these is THC, which also leads to the signs that someone has been smoking pot: the memory loss and random thoughts, as well as the unsteady walk.

Breathing In the Smoke

Marijuana smoke is also filled with many of the same chemicals as tobacco smoke, including ammonia, hydrogen cyanide, and formaldehyde. Some of these chemicals are known to cause cancer. Most users smoke pot in a joint or water pipe, so they breathe the smoke straight into their lungs.

There's no proof that smoking marijuana causes lung cancer like cigarettes do. But people who smoke pot do show signs of damage and precancerous changes in their lungs, especially if they also smoke cigarettes. And a study published in 2013 in Cancer Causes & Control found that heavy marijuana smoking might raise the risk of lung cancer.

Pot smoking leads to other lung effects, too. "We know that patients see their doctor with more symptoms, including cough and wheezing, when they're marijuana smokers," Tetrault says.


Marijuana on the Brain

Could smoking marijuana change the way the brain works? That's what researchers are finding.

  • Brain imaging scans of heavy marijuana smokers have revealed changes in blood flow to parts of the brain involved in memory and attention. Researchers have also noted differences in the size and shape of the thalamus, the part of the brain that's involved in consciousness and information processing.
  • Heavy pot smokers in studies score lower than non-users on tests of memory, attention, and learning. The more they smoked, the worse they did.
  • The effects of smoking pot may be even more pronounced in teenage smokers than adults, because teens' brains are still developing.
  • Regular users are more likely to be diagnosed with schizophrenia, especially when they have a family history of the condition.

Other Health Effects

Marijuana can have effects beyond the lungs and brain. These include:

Is It Addictive?

People who stop smoking pot don't have the same withdrawal symptoms -- like anxiety and sweating -- they'd get if they were addicted to a drug like heroin. Yet those who use marijuana all the time can have a hard time stopping.

The condition of marijuana dependence does exist, studies show. Some long-term, near-daily users seek treatment to quit, yet they keep smoking marijuana, despite its social, psychological, and physical effects. They also mention effects such as relationship and family problems, low energy and self-esteem, memory problems, and low life-satisfaction.  Learn more about what happens when marijuana addiction goes untreated.

Marijuana can also lead to other addictions¸ especially in people who start smoking at a young age. That's why it's sometimes called a "gateway drug." One study found that young people who smoke marijuana are more likely to abuse other drugs, including prescription opioids, in the future.

Eat It Instead?

Although smoking is the most common way to use marijuana, some people bake it into a brownie or other food. Eating pot might spare you the lung effects of this drug, but that doesn't mean it's safe.

Because it takes a while for your body to digest marijuana, you might not feel the effects very quickly. If you keep eating more to get high, you could overdose. Signs of an overdose include sudden anxiety and panic.

Marijuana hangs out in the body longer when it's eaten than when it's smoked, so you could feel the hangover effects -- like a dry mouth and bloodshot eyes -- well into the next day.

The Bottom Line on Marijuana

There is some evidence that occasional pot smoking can have harmful effects on the body, although the science is still too new to prove anything.

"We're learning new things every day," says Matthew J. Smith, PhD, a research assistant professor at the Northwestern University Feinberg School of Medicine. "Before we can really understand the effects of marijuana, further research is needed."

Show Sources


U.S Department of Health and Human Services: "Results from the 2012 National Survey on Drug Use and Health. Summary of National Findings." 

Gallup: "For First Time, Americans Favor Legalizing Marijuana."

CNN: "Marijuana is safer than alcohol." 

Hall, W. The Lancet, October 2009. 

Moir, D. Chemical Research in Toxicology, February 2008. 

Pletcher, M.J. JAMA, January 2012. 

Jeanette Marie Tetrault, MD, FACP, assistant professor of medicine at the Yale University School of Medicine.

National Institute on Drug Abuse: "Is marijuana addictive?"

Fiellin, L. Journal of Adolescent Health, February 2013.

Mehra, R. Archives of Internal Medicine, July 2006. 

Smith, M.J. Schizophrenia Bulletin, December 2013. 

Matthew J. Smith, PhD, research assistant professor in the Department of Psychiatry and Behavioral Sciences at the Northwestern University Feinberg School of Medicine.

National Institute on Drug Abuse: "DrugFacts: Marijuana." 

Li, Mu-Chen. Epidemiologic Reviews, October 2011. 

Columbia University: "Go Ask Alice! Eating Marijuana."

National Institute on Alcohol Abuse and Alcoholism: "Alcohol's Effects on the Body." 

National Council on Alcoholism and Drug Dependence, Inc.: "Drinking and Driving." 

National Institute on Alcohol Abuse and Alcoholism: "Examples of Alcohol's Effect on Organ Function."

March of Dimes: "Alcohol During Pregnancy." 

Pletcher, M.J. JAMA, January 2012.

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