Ayahuasca: alkaloids, plants & analogs
Section 1 :
Dosages & activity
120 mg of harmine base (1.5 mg per kilogram is a better view) is usually recommended as the absolute lower limit for full oral activation of DMT. Some Ayahuasca brews have been found to contain slightly in excess of 400 mg per dose.
Increasing the dosage of the b-carboline only increases the somatic effects (including nausea) as the psychoactive effects which are said to occur do so only at much higher levels than a person is likely to consume (or want to consume). The best idea is to use the least amount of harmine that will work and regulate the effect by varying the amount of active entheogen that is used.
Bioassays by Ott and others have helped delineate the lowest threshold. Ott determined that 141 mg of harmine hydrochloride [equivalent to 47 mg of harmine hydrochloride] was sufficient to enable full oral activation of DMT but 40 mg of the free base [equivalent to 120 mg of harmine base; in Ott's case 1.5 mg./ kg.] was not .
Harmaline is said to be about 1.5 times as active as harmine but 100 mg of it has been determined to be the amount required to orally activate DMT . Others have claimed it twice as potent as harmine in activity. It might be noted that activity and the ability to orally potentiate or activate tryptamines can not automatically be assumed to be synonymous.
It has been stated by Naranjo 1967 that 300 mg of racemic tetrahydroharmine is equal to 100 mg of harmaline. This is based on a single experiment involving one person. So far as can be determined, no attempts have been made to determine whether it is effective in the oral activation of tryptamines but the oral activity of UdV hoasca determined to contain only subthreshold amounts of harmine as well as tetrahydroharmine suggests that it may be effective for this purpose. Its presence as the lone alkaloid in an ayahuasca additive that was reported active, as noted by Shulgin stresses the need for evaluation, as does the aforementioned analysis of UdV prepared ayahuasca which contained a substantial but subthreshhold dose of harmine co-occurring with a similar amount of tetrahydroharmine.
Despite its decent MAOI (and stronger sedative) properties, 250 mg of Harman did not activate several more-than-adequate dosages of DMT. Attempts to determine whether a larger amount would work have not been performed.
Despite assorted armchair assumptions online to the contrary, it seems unlikely that Tetrahydroharman would be any more effective than harman and apparently this known weak MAOI has not been specifically evaluated for this purpose.
Tetrahydroharmine is suggested to be effective for this purpose based on its sometimes high presence in effective UdV hoasca brews containing what are thought to be subthreshold levels of harmine, but Callaway has pointed out that there are several potential routes for it to affect the activity of DMT (see comments elsewhere here) and it may show psychoactivity of its own (which is another subject in serious need of real study).
In experiments designed to ascertain relative MAO inhibiting abilities, Udenfriend and associates (1958) determined harmine and harmaline to be about equal in activity with harman and tetrahydroharmine less potent, McKenna and coworkers (1984a) obtained similar results, as did Buckholtz & Boggan 1977, but found harmaline to be slightly more active than harmine. In contrast to this is McIsaac & Estévez 1966 who reported harman as having the greatest activity (and norharman having yet greater activity). McIsaac & Estévez used calf liver mitochondrial homogenates, while Udenfriend and McKenna had both used rat liver homogenates and Buckholtz & Boggan: rat liver and brain homogenate. [Comments adapted from Ott 1994]
It is this author's recommendation that if planning on using an MAOI to orally activate DMT, that harmine and harmaline be used as the substances of choice. They are proven to be reliable and short-acting; their MAO inhibition is totally and readily reversible once the dosage wears off.
Both are reasonably safe when used as directed, are fairly readily available, relatively inexpensive and have both well known dosage levels and parameters of effects. Tetrahydroharmine may be useful as well but its activity needs to be much better defined.
The pharmaceutical product moclobemide  has also been determined to be effective for this purpose and is preferred by some due to the ability of the user to precisely regulate the dosage and also its lack of any sedative side-effects. It similarly is a reversible MAOI but its potential for adverse drug interactions is unclear.
Cultivation of B. caapi as a plant grown outdoors but overwintered indoors and used for leaf production (and young stem trimmings) is a relatively unexplored area that the literature suggests would be quite fruitful. While the alkaloid content is much lower than P. harmala, the growth of B. caapi under good conditions is phenomenal.
The leaf is said by at least one ayahuascero not to be used for making ayahuasca due to it not tasting right. In what data points we have the alkaloid levels of the bark seem to be less than those of the leaves (or the roots or the seeds in increasing order) but more analytical investigations are needed. Even with greenhouses, northern climate cultivation of B. caapi for stem bark is realistically viable only for relatively small amounts; adequate for an individual but not feasible for commercial scale production. The standard recommendation among growers is that harvest of stem-bark should not begin until the material reaches 7 years of age but the reasons for this are unclear.
While reliably within a useful range, the actual levels of total alkaloid found within P. harmala seeds varies from 2-7% according to Ott 1993. To account for the variability, Ott recommended 3 to 4 grams of the seeds be used for oral activation. (approx. 1 tsp.)
Marion reported quantitative assay yields of 2.09% harmaline and 1.60% harmine. Hasenfratz recovered 1.5% harmaline and 1% harmine.
Even if present at levels as low as 1%; this will still yield 4.5 grams of harmine for each pound of P. harmala. (A pound of green seeds testing as well as Degtyarev's would yield over 19 grams of harmine)
Purchased in bulk from seed suppliers, pound prices range from $20 on up. [Seeds for dye purposes generally command lower prices than do viable seeds intended for sowing.] If obtained as incense from Middle Eastern groceries under the name Esphand it will cost between $6 and $12 a lb. retail.
A pound of seeds will average out to around two dozen mao inhibiting doses capable of orally activating tryptamines even if the material was the weakest available (over a hundred average dosages if the green seeds mentioned above were used).
There are vast differences in alkaloids depending on source of material and also on time of harvest. [See elsewhere here for a few examples.]
Extracting and using pure alkaloids has the advantage of enabling one to always being able to take a known dose. This is something which is not always possible when dealing with either commercial or wild plant material.
It cannot be stressed enough that just because a particular plant species has been reported in the literature to yield a given percentage, this does not mean that a given sample of the same species will contain an identical concentration or even the identical alkaloids! Either or both of these points are true for virtually any plant and plant alkaloid.
Contrary to popular belief, by themselves, MAO inhibitors like harmine are not hallucinogenic , Ott best described them when he said that they produced a "Valium like sedation."  This effect persists for a couple hours after the tryptamines wear off .