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Updated November 10, 2009

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Definition: THC is the active substance in marijuana that produces a psychoactive reaction. Also known as delta-9-tetrahydrocannabinol, THC is a naturally occurring chemical substance in the marijuana plant.

Studies have shown that THC has medicinal properties that can relieve the symptoms of some diseases and conditions, as well as the side effects of their treatment. Cancer patients have used marijuana medicinally to find relief from the pain, nausea, and loss of appetite that can be caused by treatment.

Synthetic THC is available as a pharmaceutical-grade drug called Marinol (dronabinol).

Also Known As: tetrahydrocannabinol

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