What is the main active chemical in marijuana?

The main active chemical in marijuana is tetrahydrocannabinol, commonly abbreviated THC.

THC is a cannabinoid produced by the cannabis plant and is chiefly responsible for marijuana's intoxicating or psychoactive effects. In scientific and medical contexts, “THC” usually means delta-9-tetrahydrocannabinol, or Δ9-THC. It affects the nervous system mainly by partially activating CB1 cannabinoid receptors in the brain and CB2 receptors elsewhere in the body.

Fresh cannabis primarily contains tetrahydrocannabinolic acid (THCA), a non-intoxicating precursor. Heat or aging removes a carboxyl group from THCA in a process called decarboxylation, producing THC. That is why heating cannabis changes its chemical effects and why preparation matters.

THC is not the same as cannabidiol (CBD), another well-known cannabis compound. CBD is not considered intoxicating in the same way, although cannabis products can contain both substances. Researchers Raphael Mechoulam and Yehiel Gaoni first isolated THC from cannabis in 1964. THC is also used medically in preparations such as dronabinol, including treatments for chemotherapy-related nausea and appetite loss.