Why does DMT have little to no effect when taken orally by itself?

DMT and monoamine oxidase (MAO) DMT is rapidly broken down by monoamine oxidase (MAO) enzymes, particularly MAO-A, which are abundant in the gut and liver. As a result, when DMT is taken orally on its own it is almost completely metabolized before it can reach the brain, producing little or no psychoactive effect. This rapid enzymatic degradation is the main reason pure oral DMT is inactive, while combinations that inhibit MAO (as in traditional ayahuasca) allow it to become active.