Donald Tashkin's is a tale cannabis pushers like to repeat. The physician and professor at UCLA's David Geffen School of Medicine set out to prove — via a study funded by the National Institutes on Drug Abuse — that marijuana is bad for you. Instead, a long-term study found no solid link between marijuana use and lung cancer.
Similar findings were repeated all over the world. In a review of studies on marijuana's effect on the lungs, published in the June issue of the Annals of the American Thoracic Society, Tashkin concludes that compared to tobacco smoking, heavy marijuana use has "relatively small and far lower" risks.
It's worth remembering that this is not a new development — Tashkin's long-term study was published in 2006. And well before that, in the 19th century, when cannabis tinctures and other marijuana medicines were sold in pharmacies, doctors were prescribing marijuana as a treatment for asthma patients.
There are similarities between tobacco and marijuana that most cannabis advocates don't like to admit: similar levels of ammonia and other carcinogens, and marijuana smokers inhale about four times the tar, Tashkin notes. Yet several long-term studies found no positive association between marijuana use and lung cancer. And others found no positive link between marijuana use, even heavy, long-term use, and chronic obstructive pulmonary disease. (Marijuana use does have deleterious effects, but they are short-term, like bronchitis, which goes away after the pipe is put away.)
The key appears to be in the THC. Marijuana's main psychoactive ingredient has also shrunk tumor cells in lab rats. It's possible that the THC encourages lung tissue to die before it can become cancerous, Tashkin suggests.
One of marijuana's most confounding effects is bronchodilation, or expansion of the lungs when exposed to marijuana smoke. Cigarette smoke, by contrast, leads to bronchoconstriction, or narrowing of air passages.
Maybe that's what the 19th century pot docs were after. And perhaps there will be some modern-day folks who catch on to this.