Research into the health effects of cannabis have yielded a mixed bag (so to speak) of results. Predictably, studies that have looked for harmful effects have found them. But there are also beneficial effects, the most surprising of which involve the ability of cannabinoids to combat cancer.
Of course, smoke (of any kind) is not good for you, and the available evidence (see this study and also this review) seems to show that smoking marijuana brings roughly the same increase in lung-cancer risk as smoking tobacco. But that's not where the story ends.
In one two-year study involving THC, a dose-related decrease in the incidence of hepatic adenoma tumors and hepatocellular carcinoma was observed in mice, and decreased incidences of benign tumors (polyps and adenomas) in mammary gland, uterus, pituitary, testis, and pancreas were noted in rats. In another study, delta-9-THC, delta-8-THC, and cannabinol were seen to inhibit growth of lung adenocarcinoma cells in vitro and in vivo.
In yet another study, cannabinoids reduced the viability of hepatocellular carcinoma (HCC) cells in vitro and demonstrated antitumor effects in HCC subcutaneous xenografts in mice.
An in vitro study of the effect of cannabidiol on programmed cell death in breast cancer cell lines found that the chemical induced programmed cell death and inhibited survival of both estrogen receptor–positive and estrogen receptor–negative breast cancer cell lines, inducing apoptosis in a concentration-dependent manner while having little effect on nontumorigenic mammary cells.
Cannabidiol has also been demonstrated to exert a chemopreventive effect in a mouse model of colon cancer.
For a list of 37 studies showing beneficial effects of cannabis, see the National Cancer Institute's page on cannabis.
How is it that cannabinoids are able to fight cancer? Apparently, THC and its relatives are able to facilitate apoptosis (programmed cell death) in neoplastic tissues, hastening the death of cancer cells. But also, cannabinoids exert an anti-inflammatory effect, and it is well known that anti-inflammatory drugs tend to have anti-cancer properties.
It should be stressed that the studies showing positive results did not use smoke as a delivery mechanism. If you decide to light up, you're probably driving your cancer odds in the wrong direction.