Marijuana Smoke and Cancer Risk

The Abramson Cancer Center of the University of Pennsylvania
Last Modified: May 8, 2013

Question

Does smoking marijuana cause cancer?

Answer

Carolyn Vachani RN, MSN, AOCN, OncoLink's Nurse Educator, responds:

This is an interesting question. There are a few things we know about marijuana smoke:

It contains several of the same substances in cigarettes that cause cancer.
Benzopyrene, a cancer-causing agent, is present in higher concentrations in marijuana than in cigarettes.
In relation to cigarette smoking, marijuana smoking may involve the inhaling of 3 times the amount of tar and 30% more of this tar is retained in the lungs.
When pathologists looked at the lungs of deceased marijuana smokers, they saw many abnormalities. The upper respiratory tract damage was similar to a pack-a-day cigarette smoker.

Despite all of this, the research studies done to date have not found an increase risk of cancer in marijuana smokers. Most of the studies were very small, and many were poorly designed, making them not very reliable. It is difficult to do a study accurately when you are asking people to self-report their marijuana smoking habits, particularly when the substance in question is illegal in most countries. Some researchers have proposed conducting studies in countries where marijuana is legal, but this has yet to be done. The other factor is that, unlike cigarette smokers, most marijuana smokers do not smoke chronically or in the volume that cigarette smokers do, so focusing on the groups that are chronic marijuana users may be more helpful in studies. The bottom line is, researchers agree that there is most likely some cancer-causing effect to smoking marijuana, but they have yet to quantify this with clinical studies.