Tetrahydrocannabinolic acid (THCA) is a precursor of tetrahydrocannabinol (THC), the main psychoactive substance found in the cannabis plant. THCA is found in variable quantities in fresh, undried cannabis, but is progressively decarboxylated to THC with drying, especially under intense heat when smoked or cooked. THCA is not directly used but its presence is commonly analyzed for cannabis screening. THCA shows a similar metabolism as THC in humans. THCA is thought to be completely converted into THC but it has been found in the urine and blood of cannabis consumers in the 10-20 ng/ml range. The pharmacological actions of THC involve the binding and activation of cannabinoid receptor CB1 in the central nervous system and the CB2 receptor in immune cells. THC is a partial agonist at these receptors and therefore they are not fully activated.