THC, which is short for Tetrahydrocannabinol, is a psycho-active substance in hemp plants. You can say that it is the most important or strongest psychoactive substance in the plant. In addition to being an intoxicant, THC has been used for the alternative treatment of many diseases over time, and more recently also as a medically recognized agent , in the form of medical cannabis, e.g. THC hemp oil for cancer and the relief of side effects such as nausea after chemotherapy.