Tetrahydrocannabinol (THC), molecular model. THC is the principal psychoactive constituent of cannabis. Cannabis-derived drugs may be medically useful due to their ability to relieve pain and nausea, increase appetite and control muscle spasms. They may be beneficial to patients with multiple sclerosis, AIDS, cancer and chronic pain from nerve damage. The molecular formula for THC is: C21.H30.O2. Atoms (spheres) are colour-coded: carbon (light grey), hydrogen (white), and oxygen (red).