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Toxicity:
(?) See comments under harmine.

Dosage:
Naranjo 1967 reported the racemate to be orally active and 300 mg as equivalent to 100 mg of harmaline [Note 1].
   Psychotropic above 3 mg/ kg iv or 12 mg/ kg oral. Ott 1996 citing Naranjo 1967.

Duration:
(?) See harmine. Duration and metabolism are likely different from harmine.

Pharmacological activity:
It must be stressed that the actual activity of THH is presently poorly defined; at best.
   MAOI. (Less active than harmine or harmaline according to most; equipotent according to some. Needs more work.)
   EC₅₀ as an MAOI is 8 x 10^-8 M (Interestingly the level of THH required for MAOI activity is above the levels reached by UdV members ingesting orally active ayahuasca) McKenna et al. 1986
Vasodilator. Raymond-Hamet 1941
Tranquilizer. Usdin & Efron 1979.
Claimed to show ‘activity' if it is smoked or if it is administered after MAO inhibition, but, oddly, no further details, supportive data or even references were included. Callaway 1995.
In need of much greater work to define its activity. Shulgin & Shulgin 1997
THH may act as a weak 5-HT uptake inhibitor and MAOI. It may also play no role in inhibiting MAO but rather have its own activity potentiated by MAO inhibition.
   Or THH may prolong the half-life of DMT by blocking its reuptake and preventing its inactivation by the MAO located in the mitochondria within the neuron.
   Or it may show psychoactive effects of its own by blocking the reuptake of serotonin into the neuron; thus resulting in a higher level of serotonin within the synapse which may "attenuate the subjective effects of orally ingested DMT by competing with it at post-synaptic receptor sites"
   Its action seems to be at least partly independent of the activity of harmine & MAO inhibition.

Callaway et al. 1999 & McKenna et al. 1998

Tetrahydroharmine was shown to be almost ineffective against bacteria but did inhibit the fungus Epidermophyton floccosum (at 500 µg/ml). Ahmad 1992

Metabolism & pharmacodynamics:
In need of much more serious study.
   When humans are given oral ayahuasca THH levels showed a half-life of 532 minutes; taking 174 minutes to reach peak plasma levels.

McKenna et al. 1998

Pharmacokinetics:

• C_max (average) 91.0 ± 22.0 ng/ml
• T_max (average) 174.0 ± 39.6 min.
• T_1/2 (average) 531.9 ± 290.8 min.
• (Dosage of 2.14 mg/kg oral) Callaway et al. 1999