Tetrahydrocannabinol, or THC, the active psychedelic ingredient in cannabis, has known anti-tumoral effects, Donald P. Tashkin, M.D., a pulmonologist at UCLA who has studied marijuana for more than 30 years, told weather.com. “It’s been shown by a large number of investigators to [reduce] growth of brain, lung, breast, prostate and thyroid cancer cells in animal models,” he said.