Phenethylamine, or β-phenylethylamine or 2-phenylethylamine, is an alkaloid and monoamine. Phenethylamine also has a constitutional isomer α-phenylethylamine (1-phenylethylamine), which has two stereoisomers: (R)-(+)-1-phenylethylamine and (S)-(-)-1-phenylethylamine. In the human brain, 2-phenethylamine is believed to function as a neuromodulator or neurotransmitter (trace amine). Phenethylamine is a natural compound biosynthesized from the amino acid phenylalanine by enzymatic decarboxylation. It is also found in many foods such as chocolate, especially after microbial fermentation. However trace amounts from food are quickly metabolized by the enzyme MAO-B, preventing significant concentrations from reaching the brain.^[2][3]

Phenylethylamine is a precursor to the neurotransmitter phenylethanolamine.^[4]

D-Phenylalanine increases the brain content of phenylethylamine.^[5] Phenylethylamine has been shown at least indirectly to satisfy the four main criteria required to demonstrate that a neuroamine sustains mood and that its deficit can be responsible for depression.^[5] Antidepressant drug therapy is an indirect (pharmacological) "replacement" of brain phenylethylamine.^[5]

Substituted phenethylamines are a broad and diverse class of compounds that include stimulants, hallucinogens, entactogens, anorectics, bronchodilators, and antidepressants.