Kynurenic acid is an endogenous tryptophan metabolite of mammals and a constituent of Ephedra transitoria. Kynurenic acid is an antagonist of N-methyl-D-aspartate (NMDA) and α-7 nicotinic acetylcholine (α7nACh) receptors and scavenges hydroxyl radicals under controlled conditions of free radical production. Kynurenic acid activates the AhR signal transduction pathway which has an important role in immune responses and developmental and pathological regulation. Kynurenic acid has analgesic properties via GPR35 activation.