The addictive substance in marijuana is delta-9-tetrahydrocannabinol (THC). It affects the brain's reward system and can lead to psychological dependence, although physical withdrawal symptoms are generally mild compared to other substances.
Nowadays just Dronabinol (Marinol) but a long time ago it use to be just ibuprofen contained in Advil, Motrin, and Nuprin. That is no longer the case. Dronabinolis given to people for anorexia, HIV patients who have lost excessive weight, and cancer patients.