Shamanism is a form of traditional medicine, common to a number of cultures including Latin America, Native American, and Oceania. It does not include traditional African witch doctor medicine. An intermediate stage between polytheism and monotheism, which assumes a "Great Spirit", with lesser deities subordinated. With the beginnings of shamanism there was the advent of the medicine man or witch doctor, who assumed a supervisory relation to disease and its cure. A shaman acts as a kind of mediator between the world of the dead and the world of the living, and between the animals and the human society. He also protects his or her group against threats from evil spirits or other shamans. Formally, shamanism is a religion of Ural-Altaic peoples of Northern Asia and Europe, characterized by the belief that the unseen world of gods, demons, ancestral spirits is responsive only to shamans. The Indians of North and South America entertain religious practices similar to the Ural-Altaic shamanism. The word shaman comes from the Tungusic (Manchuria and Siberia) saman, meaning Buddhist monk. The shaman handles disease almost entirely by psychotherapeutic means; he frightens away the demons of disease by assuming a terrifying mien. 
Shamanic states of consciousness
Michael Harner, who was controversial as a "participant observer" rather than an uninvolved anthropologist, described certain shamanistic widely diverse cultures that are unlikely to have had contact. These include the belief in a ordinary state of consciousness (OSC) that corresponds to physical reality, and a shamanic state of consciousness (SSC) that is an alternate reality. The different cultures share a common idea of a shamanic journey between the OSC and SSC. 
Some modern shamans consider that individual or cultural belief systems, biochemistry, or  Any or all of these can apply. The belief systems, for example, may include a feeling of community, which allows additional means of interpersonal community, especially in culturally-specific ritual.
Achieving the journey
The journey is often facilitated with plant-derived drugs. According to a literature "nonordinary reality" or SSC has been monitored and does show altered brain activity, which is different from other trance and seizure states. The nature of this brain function may indicate there is a genetic ability to enter the SSC, with or without drugs, which would be consistent with indigenous beliefs that not all individuals are capable of the shamanic journey. 
Drumming or chanting, either in addition to or instead of drugs, may be used.
While both Western psychotherapy and shamanism have made use of psychoactive drugs, the similarity is superficial; fundamentally different concepts are employed. It should be understood that only hallucinogens are being considered within the scope of Western psychopharmacology, not tranquilizers, antidepressants, etc. In both the Western and shamanistic traditions, 
The differences in models include beliefs, and the ability to perceive,
- multiple realities ("worlds") that can be explored in expanded states of consciousness
- "spirits," the beings one encounters in dreams and visions, are just as real as the physical organism
Again depending on the particular tradition, the shaman may journey alone, or guide the patient in his journey. While such beliefs and perceptions are inconsistent with Western psychedelic-aided psychotherapy, they are consistent with various spiritual practices, including western religious trance ecstasy.
Four paradigms can be considered, which have a strong differentiation from the Western and traditional views.
- psychedelic psychotherapy within the standard Western paradigm--here the drug is used to amplify and intensify the processes of internal self-analysis and self-understanding
- shamanic rituals of healing and divination, which involve primarily the shaman or healer taking the medicine in order to be able to "see" the causes of illness and know what kind of remedy to apply
- syncretic folk religious ceremonies, in which the focus seems to be a kind of community bonding and celebratory worship
- "hybrid shamanic therapeutic rituals," which incorporate some features of the first two traditions.
- Anonymous (2021), Shamanism (English). Medical Subject Headings. U.S. National Library of Medicine.
- quoted in MeSH From Garrison, An Introduction to the History of Medicine, 4th ed, p22
- Harner, Michael, The Way of the Shaman
- Jade Wah'oo Grigori (Autumn Equinox 2002), Earth-Shaman Articles, Insights and Ravings
- Wright PA. (1989 Jan-Mar), "The nature of the shamanic state of consciousness: a review.", J Psychoactive Drugs 21 (1): 25-33
- Jade Wah'oo Grigori, Shamanic Drumming
- Metzner R. (1998 Oct-Dec), "Hallucinogenic drugs and plants in psychotherapy and shamanism.", J Psychoactive Drugs 30 (4): 333-41