Banisteriopsis caapi is an enormous South American liana (woody vine) in the plant family Malpighiaceae with tiny pink flowers and a 3-winged maple-like fruit (samara). A hallucinogenic drink made from the bark of this and the related B. inebrians has long been widely used by Indians in the western Amazon (Brazil, Colombia, Peru, Ecuador, and Bolivia). The drink is intensely bitter and nauseating. Some tribes add other plants to the mix as well (e.g., B. rusbyana, Datura suaveolens, Brunfelsia sp., and  especially Psychotria viridis) and in some parts of the Orinoco the bark is simply chewed. In some areas, the plant may be used as a component in a snuff (e.g., Rodd 2002). Both B. caapi and the concocted drink may be referred to by the Quechua name "ayahuasca", among other names, but for clarity many authors use this name to refer only to the concoction. (Schultes 1976)

According to Schultes (1976), the effects of drinking ayahuasca range from a pleasant intoxication to violent reactions with sickening lingering effects, depending on the preparation and the person taking the drug mixture. Usually there are visual hallucinations and with larger doses there may be nightmarish visions and a sense of reckless abandon. Ayahuasca is taken by traditional healers in some tribes to help them diagnose and treat illnesses; it may also be taken as part of religious ceremonies or as part of a ceremony initiating boys into manhood.

Ayahuasca is a rich source of harmine, tetrahydroharmine (THH), and, to a lesser extent, harmaline. Some of the plants often added in addition to B. caapi, notably Psychotria viridis, contribute dimethyltryptamine (DMT) to the mix. Importantly, the harmala alkaloids in B. caapi are short-acting reversible monoamine oxidase (MAO) inhibitors, which prevent DMT from being metabolized before reaching the brain. In some areas, however, B. caapi is used alone, without adding any DMT-containing plants (Rodd 2008). The alkaloid profiles of ayahuasca from different sources are highly variable (Rivier and Lindgren 1973; Callahan 2005).

Dobkin De Ríos (1970) investigated the use of ayahuasca for both purposes of healing and witchcraft by the urban poor in an Amazon city slum iin Iquitos, Peru, in the mid-20th century. The religious ceremonial use of ayahuasca remains common today not only in Amazonian indigenous cultures, but also  in urban areas throughout much of South America (Callaway 2005). In recent years it has become increasingly common in Europe and North America as well (Tupper 2009). Harris and Gurel (2012) studied the subjective experiences of ayahuasca users in North America. The global spread of ayahuasca’s use in recent years has led to a number of legal cases pitting religious freedom against national drug control laws. Members of some religious groups use ayahuasca at least twice a month (Bouso et al. 2012). Various legal, religious, philosophical, and policy issues arising from the global spread of ayahuasca use are discussed by Tupper (2008).

Bouso et al. (2012) found no evidence of psychological maladjustment, mental health deterioration or cognitive impairment among a group of regular users of ayahuasca, although they acknowledge possible weaknesses in their study, most notably the possibility that their ayahuaca-using group was self-selected to include individuals who had positive experiences with ayahuasca.

With a nod to a group of researchers who published a dense in the journal Science of the right of the Native American Church to use peyote in its religious rites, Anderson et al. (2012) published an editorial addressing the right to use ayahuasca in religious practices.

Rivier and Lindgren provide a fascinating account of the use of ayahuasca in the Peruvian Amazon, as well as their own experiences with it (Rivier and Lindgren 1972). Flores and Lewis (1978) offer another personal account.