Fenclonine, also known as para-chlorophenylalanine (PCPA), acts as a selective and irreversible inhibitor of tryptophan hydroxylase, which is a rate-limiting enzyme in the biosynthesis of serotonin.

[2] It has been used experimentally to treat carcinoid syndrome, but the side effects, mostly hypersensitivity reactions and psychiatric disturbances, have prevented development for this use.

Tryptophan hydroxylase activity can be detected neither in cell bodies or nerve terminals.

Aromatic L-amino acid decarboxylase (AADC) levels were at no time affected.

[4] It is used in scientific research in humans[5] and animals[2] to investigate the effects of serotonin depletion.