Weed, doobie, and pot are just a few of the many slang terms used for the drug marijuana. In the United States, marijuana is the drug that most people get arrested for. Studies show that long term use of marijuana does not increase a person's cancer risk.
Cannabis, also known as marijuana (from the Mexican Spanish marihuana), and by other names, is a preparation of the Cannabis plant intended for use as a psychoactive drug and as medicine. Pharmacologically, the principal psychoactive constituent of cannabis is tetrahydrocannabinol (THC); it is one of 483 known compounds in the plant, including at least 84 other cannabinoids, such as cannabidiol (CBD), cannabinol (CBN), tetrahydrocannabivarin (THCV), and cannabigerol (CBG).
Generally it's thought to diminish the effects.