Kynurenine is a metabolite of the amino acid L-tryptophan used in the production of niacin. Kynurenine is synthesized by the enzyme tryptophan dioxygenase, which is made primarily but not exclusively in the liver, and indoleamine 2,3-dioxygenase, which is made in many tissues in response to immune activation. Kynurenine and its further breakdown products carry out diverse biological functions, including dilating blood vessels during inflammation and regulating the immune response. Evidence suggests that increased kynurenine production may precipitate depressive symptoms associated with interferon treatment for hepatitis C. Cognitive deficits in schizophrenia are associated with imbalances in the enzymes that break down kynurenine. Kynurenine production is increased in Alzheimer's disease and cardiovascular disease where its metabolites are associated with cognitive deficits and depressive symptoms.

Peforelin, a decapeptide analog of the natural gonadotropin acts as the natural gonadotropin-releasing hormone (GnRH) agonist. This drug under the brand name Maprelin is used in veterinary to stimulate the reproductive function of sows after weaning piglets in the syndrome of sexual depression.

Aldose reductase-IN-1 is an inhibitor of aldose reductase