Marijuana - often called pot, grass, reefer, weed, herb, mary jane, or mj - is a greenish-gray mixture of the dried, shredded leaves, stems, seeds, and flowers of Cannabis sativa, the hemp plant. Most users smoke marijuana in hand-rolled cigarettes called joints, among other names; some use pipes or water pipes called bongs. A marijuana user may experience pleasant sensations, colors and sounds may seem more intense, and time appears to pass very slowly. The user's mouth feels dry, and he or she may suddenly become very hungry and thirsty. His or her hands may tremble and grow cold. The euphoria passes after awhile, and then the user may feel sleepy or depressed. Occasionally, marijuana use produces anxiety, fear, distrust, or panic. THC, the main active ingredient in marijuana, produces effects that potentially can be useful for treating a variety of medical conditions. It is the main ingredient in an oral medication that is currently used to treat nausea in cancer chemotherapy patients and to stimulate appetite in patients with wasting due to AIDS. Marijuana is the Nation's most commonly used illicit drug. More than 83 million Americans (37 percent) age 12 and older have tried marijuana at least once. Urine detection limits vary widely depending on the frequency of the drug use. In chronic users, the THC can remain detectable in the urine for up to 28 days.