[3][6] The coincident occurrence of β-carboline alkaloids and serotonin in Peganum harmala indicates the presence of two very similar, interrelated biosynthetic pathways, which makes it difficult to definitively identify whether free tryptamine or L-tryptophan is the precursor in the biosynthesis of harmine.

Decarboxylation of L-tryptophan by aromatic L-amino acid decarboxylase (AADC) produces tryptamine (I), which contains a nucleophilic center at the C-2 carbon of the indole ring due to the adjacent nitrogen atom that enables the participation in a Mannich-type reaction.

Rearrangements enable the formation of a Schiff base from tryptamine, which then reacts with pyruvate in II to form a β-carboline carboxylic acid.

[4] Oral or intravenous harmine doses ranging from 30 to 300 mg may cause agitation, bradycardia or tachycardia, blurred vision, hypotension, paresthesias.

[4] The psychoactive ayahuasca brew is made from B. caapi stem bark usually in combination with dimethyltryptamine (DMT) containing Psychotria viridis leaves.

[13] Harmine is currently the only known drug that induces proliferation (rapid mitosis and subsequent mass growth) of pancreatic alpha (α) and beta (β) cells in adult humans.

[14] These islet sub-cells are normally resistant to growth stimulation in the adult stage of a human's life, as the cell mass plateaus at around age 10 and remains virtually unchanged.

A 2024 Phase 1 clinical trial investigating pharmaceutical-grade harmine hydrochloride in healthy adults found that the maximum tolerated dose (MTD) is approximately 2.7 mg/kg body weight.

No serious adverse cardiovascular effects were observed at any dose tested (up to 500 mg), though rare instances of transient hypotension occurred during episodes of vomiting.

[17] In addition to B. caapi, at least three members of the Malpighiaceae contain harmine, including two more Banisteriopsis species and the plant Callaeum antifebrile.

[20][21] In 1905, the Colombian naturalist and chemist, Rafael Zerda-Bayón suggested the name telepathine to the then unknown hallucinogenic ingredient in ayahuasca brew.

[3][22] In 1923, the Colombian chemist, Guillermo Fischer-Cárdenas was the first to isolate harmine from Banisteriopsis caapi, which is an important herbal component of ayahuasca brew.

[23] In 1925, Barriga Villalba, professor of chemistry at the University of Bogotá, isolated harmine from B. caapi, but named it "yajéine",[13] which in some texts is written as "yageine".

With the assistance of Professor Robert Robinson in Manchester, Elger showed that harmine (which was already isolated in 1848) was identical with telepathine and yajéine.