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Dr. Dean

Why Doesn't Smoking Marijuana Cause Cancer?

Posting Date: 11/25/1999

Original broadcast date: July 15, 1999

Haroon: There's such a big to-do about smoking and cancer, but people smoke marijuana and call that medicinal.

I was told that the heat from the smoke causes cancer among tobacco smokers. But then wouldn't smoking marijuana cause cancer, too?

Dr. Dean: Haroon, this is the big question about smoking. You'd think we'd know by now what part of it causes cancer, but we don't.

Some of the theories are tars, polonium, and, as you mentioned, the burning process.



The tars in cigarette smoke do contain known carcinogenic chemicals, but those chemicals don't really accumulate where lung cancer accumulates.

Polonium, which is radioactive, accumulates on tobacco leaves as a result of fertilizers. In a chronic smoker, polonium accumulates in the bronchial tree, which is also a location for cancer. If the fertilizer causes cancer, maybe non-fertilized tobacco would make a safe cigarette.

Burning vegetable matter also produces known carcinogens. But marijuana, which is certainly burning vegetable matter, doesn't seem to cause lung cancer.

You can see how difficult it is.

Burning materials, even the ones produced when we barbecue, create substances that cause cancer in laboratory animals. Those substances are more abundant in marijuana and hashish smoke than in tobacco smoke.

However, a cigarette smoker smokes 20 or 30 cigarettes a day; a marijuana smoker smokes one or two at the most. The quantity might be the difference between cancer and no cancer.

One of the main reasons we can't pinpoint what part of smoking causes cancer is that cigarette companies have a big secret. An incredible loophole in the law allows them not to disclose about 500 of the ingredients in cigarettes. Maybe the burning of one of those ingredients causes lung cancer. But we can't test it, because we don't know what it is.