GR-55562 is a selective serotonin 5-HT<sub>1B</sub> and 5-HT<sub>1D</sub> receptor antagonist. It is one of several selective serotonin 5-HT<sub>1B</sub> receptor antagonists used in scientific research.
The drug is a silent antagonist of the serotonin 5-HT<sub>1B</sub> receptor, unlike the related agent GR-127935. GR-55562 has around 10-fold selectivity for the serotonin 5-HT<sub>1B</sub> receptor over the serotonin 5-HT<sub>1D</sub> receptor and has only weak affinity for a number of other serotonin receptors.
It is ineffective in attenuating MDMA-induced prosocial behavior in animals. Conversely, the serotonin 5-HT<sub>1A</sub> receptor antagonist WAY-100635 can abolish MDMA-induced prosocial behavior.