Purgatives – Many shamanic rituals and performances include the ingestion of plants that result in vomiting. Some observers, including enthusiastic participants (e.g., Gordon Wasson) have mistaken this for an unfortunate and embarrassing side effect, but the pervasive indigenous understanding is that this is the key thing: it is an absolutely necessary act of purification aided by powerful other-than-human persons. Maria Sabina, for example, insisted that if a patient or client did not vomit with the help of mushrooms, as ritualist and doctor she would have to do so. Many of the plants valued as inspirers of visions or altered states of consciousness are at least equally important as emetics (inducers of vomiting) and aids toward purity.
Historical Dictionary of Shamanism by Graham Harvey and Robert J. Wallis 2007