c21h20o2

tetrahydrocannabinol \tet`ra*hy`dro*can*nab"i*nol\
   (t[e^]t`r[.a]*h[imac]`dr[-o]*k[a^]n*n[a^]b"[i^]n[add]l), n.
   (Chem. & Med.)
   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 [Delta]1-3,4-trans isomer, also
   called {[Delta]9-THC}, with small amounts of the
   [Delta]6-3,4-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. --MI11
   [PJC]