Marijuana: A common street and recreational drug that comes from the marijuana plant: the hemp plant cannabis sativa. The pharmacologically active ingredient in marijuana is tetra-hydro-cannabinol (THC). Marijuana is used to heighten perception, affect mood, and relax. Many people think marijuana is harmless, but it is not. Signs of marijuana use include red eyes, lethargy, and uncoordinated body movements. The long-term effects may include decrease in motivation and harmful effects on the brain, heart, lungs, and reproductive system. People who smoke marijuana are also at increased risk of developing cancer of the head and neck. A pharmaceutical product, Marinol, that contains synthetic THC, is available as a prescription medication. It comes in the form of a pill (eliminating the harmful and cancer-causing chemicals present when marijuana is smoked) and is used to relieve the nausea and vomiting associated with chemotherapy for cancer patients and to treat loss of appetite in AIDS patients.