Ayahuasca, also called yage or hoasca, in popular usage (as a term for a drug), refers to a traditional South American decoction of two or more particular psychoactive plants, which in combination produce potentially profound psychedeic effects which are "greater than the sum of it's parts," as a very particular pharmacological process. In essence, coadministration of DMT along with an MAO inhibitor allows for the DMT to pass from the digestive system into the bloodstream, and from there into the brain, where it can exert it's psychedelic effects. Oral administration of DMT without inhibition of MAO causes has no discernible psychotropic effect, due to the fact that the enzyme MAO-A is present in the GI tract and leads to the total breakdown of any DMT encountered there.
The use of ayahuasca predates, probably by many centuries, the arrival of Westerners on the American continent, and was developed by the indigenous peoples there as part of their shamanic religious practices. These groups would make decoctions of several plants endemic to the regions they inhabited. Most properly the term ayahuasca refers to the vine Banisteriopsis caapi, the essential ingredient, but, rather confusingly, the term is also used in reference to psychedelic drug decoctions, or "brews." The most traditional preparations, generally speaking, use B. caapi, and, pars pro toto, the term is used for the brew itself, which is by definition a mixture of several different constituents in addition to B. caapi, one of which would typically contain DMT.
While B. caapi does have some intrinsic psychoactive, or perhaps even psychedelic, properties, the most remarkable effects of ayahuasca are due to the concomitant administration of at a number of alkaloids from at least two different plants. This is particularly remarkable because the concomitant administration of these psychoactive drugs was obviously arrived upon without any knowledge of the pharmacological processes involved, a fact which has engendered much metaphysical speculation. This speculation, while it may feed a certain need in some quarters for a supernatural paradigm upon which to base the psychedelic experience, in some ways denigrates the equally remarkable truth that even "primitive" societies can possess a great deal of botanical knowledge, and, through a "trial and error" process over generations, could arrive at a combination of plants with a truly remarkable effect, and, knowing a good thing when they saw it, pass on and perpetuate the knowledge involved in preparing it.
Generally speaking, to be properly termed ayahuasca, the brew plant most contain at least two pharmacologically and botanically distinct parts. The traditional constituents are as follows: first, a source of MAO inhibiting alkaloids, most often harmine (T14) and harmaline (T13), which role is played by B. caapi in the archetypical brew, and second, a plant which contains DMT, which will be activated, or more properly, allowed to enter the bloodstream, and have it's neuropsychopharmacology markedly changed, by the MAO inhibitor. A variety of plant sources of DMT have been used, both traditionally and more recently, as the second constituent in ayahuasca decoctions. The most traditional is Psychotria viridis, which is called "chacruna," said to derive from a word in the Quechua language meaning "to mix," as in, to mix with ayahuasca.
It is important to note that the specific constituents of the preparation can vary widely between sources, for reasons of availability, expediency, personal preference, and even deliberate deceit (there are instances where unscrupulous individuals have made "brews," so-called, containing not the expected psychoactive plants but rather one or more research chemicals holding at best a tangential chemical and pharmacological relationship to the traditional ingredients.) Even in the most traditional settings, ayahuasca has never been a set "recipe" but rather a sort of psychoactive "Stone Soup" containing any number of different plants, and thus, any number of different alkaloids.
Even the most orthodox traditional usage can turn the term "ayahuasca" into a wide umbrella. A wide variety of plants have been reported as being utilized as part of ranging from relatively equivalent DMT-containing alternatives ("Mimohuasca" using M. hostilis, etc.) to plants which contain other drugs, such as Diplopterys cabrerana (also called Banisteriopsis rusbyana), which contains 5-MeO-DMT in addition to DMT. Drugs with rather different pharmacological properties, ranging from psychedelic mushrooms, also tryptamines and also dramatically changed in their effects when administered with an MAO inhibitor (although, unlike DMT, this coadministration is not necessary in order to achieve oral activity), to atropine and scopolamine containing plants from the Nightshade family, e.g. Datura spp., which have a very different sort of "hallucinogenic," effect, more properly called a deleriant or psychotomimetic effect, and even Nicotiana rustica, a plant of the tobacco family high in nicotine.
Whether a brew may contain additional ingredients, without being advertised as such, and still be legitimately called the name, is a controversial topic which is beyond our scope, but for anyone who is considering consuming ayhausca not made by themselves, it is exceedingly important to be aware of the potential for variability. The practice of so-called "shamans" who provide individuals with doses of "ayahuasca," so called, without an explanation of the constituent ingredients, and include ingredients which go beyond the usual pair (MAO inhibitor and DMT source) is reprehensible in the extreme, not to mention potentially dangerous. We can even imagine scenarios in which it would be life-threatening.
Simple variance in the source of DMT, which is present in a wide variety of plants, is unlikely to cause major qualitative changes in the nature of the experience, provided that the quantity of plant material that is used in the brew is proportional to the concentration of the active alkaloid, as obviously otherwise an over- or under-dose of DMT would occur relative to the dose of the MAO inhibitor, resulting in an experience that is either too strong or two weak. One hypothetical concern is that if the concentration of DMT were lower than expected, relative to the amount of MAO inhibitor, an excess dose of the latter might be consumed. If this Erowid report is to be believed, such an experience can be quite physically distressing, and hallucinogenic in it's own right, however rather not in a pleasant way. Not much is known about extreme overdose of B. caapi, P. harmala, and the like, and while an educated guess might suggest that they would be safer than an overdose of, say, moclobemide or phenelzine, this is uncertain and potentially dangerous territory.
Furthermore, even if the botanical constituents are held constant, between different plant specimens of the same species, alkaloidal concentration can vary to a rather significant degree, as can the ultimate alkaloidal content of the decoction, based upon a variety of factors involved in it's preparation. One "serving" of ayahuasca may vary wildly in it's strength from one preparation to another, even prepared by the same person, and the same process, and from the same sources. This is a problem which is by no means specific to ayahuasca, but rather, is an immutable fact when dealing with plant drugs in their raw form. A skilled preparer of ayahuasca will be aware of this fact and take what amount to harm reduction measures to ensure that those consuming it do so in the safest way possible, minimizing the chance of overdose. Often, in "ceremonies" open to the general public, the initial dose is relatively weak, and a second dose, or even more, is offered once the effects of the first become apparent. It is important to note that in this instance, MAO has already been inhibited to one degree or another, so one can expect a non-linear relationship between the strength of the first and the second dose, as with the first dose it is likely that some of the DMT contained therein is lost, broken down by the GI system, before the MAO inhibitors, which are taken simultaneously, take effect. Sometimes, particularly with pharmahuasca (the combination of DMT and an MAO inhibitor in synthetic or extracted alkaloidal form rather than as raw plant material), although this deviates from the traditional model, the MAO inhibitor is taken some time prior to the DMT, in order to take advantage of the maximal inhibition of MAO.
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