Ayahuasca is an entheogenic cocktail made from sections of the vine Banisteriopsis caapi and leaves of several other plants (e.g. Psychotria Viridis or Jurema Preta). This brew contains the powerful hallucinogenic alkaloid DMT and a MAO inhibitor (Harmaline, Harmine or d-Tetrahydroharmine).
The effectiveness of this brew fluctuates dramatically from one lot to the other. The potency, strength and psychoactive effect highly depends on the skill of the shaman and the used ingredients.
It is remarkable that oral doses of those plant substances by themselves would normally not be psychoactive. It is said that Harmine/Harmaline
causes hallucinations at highly toxic levels, but taken in lower doses its best effect is tranquillizing (its baddest is that it can act as an emetic).
The oral intake of any quantity of DMT will only result in one of the most profound mind-altering experiences if a MAO inhibitor
(the Harmala alkaloids in the Banisteriopsis Caapi vine) is added - and that is precisely how Ayahuasca works.