I think the whole article is worth giving a gander, but here are the key points.

In 1974 researchers at the Medical College of Virginia, who had been funded by the National Institute of Health to find evidence that marijuana damages the immune system, found instead that THC slowed the growth of three kinds of cancer in mice — lung and breast cancer, and a virus-induced leukemia.

The DEA quickly shut down the Virginia study and all further cannabis/tumor research, according to Jack Herer, who reports on the events in his book, “The Emperor Wears No Clothes.” In 1976 President Gerald Ford put an end to all public cannabis research and granted exclusive research rights to major pharmaceutical companies, who set out — unsuccessfully — to develop synthetic forms of THC that would deliver all the medical benefits without the “high.”

“Careful MRI analysis of all those tumor-free rats showed no sign of damage related to necrosis, edema, infection or trauma … We also examined other potential side effects of cannabinoid administration. In both tumor-free and tumor-bearing rats, cannabinoid administration induced no substantial change in behavioral parameters such as motor coordination or physical activity. Food and water intake as well as body weight gain were unaffected during and after cannabinoid delivery. Likewise, the general hematological profiles of cannabinoid-treated rats were normal. Thus, neither biochemical parameters nor markers of tissue damage changed substantially during the 7-day delivery period or for at least 2 months after cannabinoid treatment ended.”

The rest can be found here.