Cinanserin (; developmental code name SQ-10643) is a serotonin 5-HT<sub>2A</sub> and 5-HT<sub>2C</sub> receptor antagonist which was discovered in the 1960s and was never marketed. It has about 50-fold higher affinity for the 5-HT<sub>2A</sub> receptor than for 5-HT<sub>2C</sub>, and very low affinity for 5-HT<sub>1</sub> receptors. The drug also inhibits the 3C-like protease of SARS-CoV-1 and SARS-CoV-2, but with much lower affinity.