ò-Carboline, also known as norharman or as 9H-pyrido[3,4-b]indole, is a tricyclic chemical compound and alkaloid. It is the parent structure of the substituted ò-carbolines, a large group of alkaloids and synthetic compounds. ò-Carboline may be thought of as a cyclized tryptamine. The compound has been found to possess a variety of pharmacological activities, including DNA mutagenic effects, imidazoline receptor interactions, serotonin reuptake inhibition, monoamine oxidase inhibition, cytochrome P450 enzyme inhibition, and inhibition of other enzymes, among others.