Trazodone is a mixed agonist and antagonist of various serotonin receptors, antagonist of adrenergic receptors, weak histamine H1 receptor antagonist, and weak serotonin reuptake inhibitor. More specifically, it is an antagonist of 5-HT2A and 5-HT2B receptors, a partial agonist of the 5-HT1A receptor, and an antagonist of the α1- and α2-adrenergic receptors. It is also a ligand of the 5-HT2C receptor with lower affinity than for the 5-HT2A receptor. However, it is unknown whether trazodone acts as a full agonist, partial agonist, or antagonist of the 5-HT2C receptor. Trazodone is a 5-HT1A receptor partial agonist similarly to buspirone and tandospirone but with comparatively greater intrinsic activity. A range of weak affinities (Ki) have been reported for trazodone at the human histamine H1 receptor, including 220 nM, 350 nM, 500 nM, and 1,100 nM.