Marijuana: Does it cause cancer?

Dear Alice,

I have been reading some of your info about marijuana. My question is: If it is so much stronger than smoking cigarettes and has some of the same ingredients, why don't we hear reports of people who have some form of cancer?

Dear Reader,

Despite the fact that humans have been growing marijuana for thousands of years, and using it recreationally in the United States since the early 20th century, its effects haven't been as thoroughly studied as those of tobacco and cigarette smoking. Most studies exploring the relationship between marijuana smoking and cancer are retrospective case-control studies (a type of observational study in which participants are selected based on their health status, such as having cancer or not having cancer). Many of these studies have shown that the individuals who have smoked marijuana have also smoked cigarettes. Given the clear correlation between cigarettes and cancer, it's difficult to tease apart the connection between cancer and marijuana, specifically. Despite the knowledge that smoke of all kinds is potentially harmful to lung health, more research is necessary to establish the relationship between marijuana and cancer. Thus far, limited studies have shown a correlation between marijuana and cancer as well as other related health consequences.

The method of marijuana consumption makes a potentially big difference in its long-term effects on the human body. Anything that heats up or vaporizes the marijuana — such as a bong, blunt, joint, pipe, or vape — has the potential to cause damage to the lungs. Marijuana smoke contains high levels of carcinogenic toxins, similar to that of tobacco smoke, but with 50 percent more benzoprene and 75 percent more benzanthracene than cigarette smoke. While there's concern that marijuana smokers inhale more deeply and keep the smoke in their lungs for a longer period than tobacco smokers, marijuana users tend to consume less of the drug overall due to its long-lasting effects. In addition to cancer, other concerns of chronic marijuana smoking include:

  • Impaired or damaged lung function
  • Bronchial irritation
  • Chronic obstructive pulmonary disease
  • Suppressed immune system — increased chance of infections including pneumonia
  • Injured cell linings and inflammation of large airway — leading to chronic cough, phlegm production, and wheezing
  • Lung hyperinflation

Despite the ongoing research efforts, it hasn't yet been established that any of these problems occur more often in marijuana smokers than in the general population. Determining whether there’s a link between marijuana and cancer isn’t exactly clear cut. Even the most well-designed population studies have failed to establish an increased risk for cancer with marijuana. First, the Schedule I classification of marijuana adds a barrier to researchers who desire to perform experiments using the drug. Additionally, most studies are performed retrospectively and are, thus, limited to what people remember and choose to report — both of which are potential sources of bias. Also, observational studies can show correlation (or lack thereof) but cause and effect is harder to establish in clinical studies. However, prospective observational studies (such as recruiting participants who either do or don't smoke marijuana already and are willing to be followed for changes in health status) may be helpful in understanding the relationship between smoking marijuana and cancer (or other health issues).

On the flip side, researchers are also looking for possible health benefits associated with marijuana use — especially among individuals with cancer. Some preclinical studies in cells and animals suggest that marijuana may have some antioxidant and anti-tumor properties that override the negative, potentially cancer-causing effects of marijuana smoke. In clinical studies, research has shown that marijuana can help manage symptoms of cancer and side effects of some cancer treatments, by:

  • Alleviating pain
  • Improving sleep
  • Stimulating suppressed appetite
  • Inhibiting nausea and vomiting
  • Reducing inflammation
  • Reducing anxiety and paranoia through actions of cannabidiol (CBD)

Ultimately, the strongest association known between marijuana and cancer is in relation to improved treatment and quality of life. Thus far, long-term marijuana smoking hasn't shown to elevate risk of multiple cancers including lung, melanoma, prostate, breast, and cervical cancers. The bottom line is that more research is needed to better understand the connection between marijuana use and potential health risks and benefits. For now, stay tuned for what information future studies might reveal. To learn more about what factors are associated with cancer, check out the American Cancer Society (ACS) and the National Cancer Institute (NCI).

Last updated Jan 07, 2022
Originally published Mar 23, 1995

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