A chemical substance (C21H20O2) which is produced by the hemp plant (Cannabis sativa), and is the physiologically active agent of dried preparations of that plant, called variously marijuana, hashish, ganja, hemp, etc.; also called THC. It causes the euphoric effect for which the preparations are smoked or chewed. It is used in medicine in a purified form as an antiemetic (an antinausea agent), especially in conjunction with chemotherapy of cancer. It occurs primarily as the D1-3,4-trans isomer, also called D9-THC, with small amounts of the D6-3,4-trans isomer detectable at about 1%. It is a controlled substance, classified as a hallucinogen, and its possession or distribution is illegal in almost all states of the United States.