The nomenclature of drugs is a critical aspect of science, since it can direct research and optimize treatment choices. Traditionally drugs acting on CNS receptors have been classified as either agonists or antagonists. Recently a new class of ligand, the inverse agonist, has been identified in some receptor systems. Inverse agonists have opposite actions to those of agonists but the effects of both of these can be blocked by antagonists. Pimavanserin is a new 5-HT2A receptor acting drug that has been given market authorization for psychosis in Parkinson׳s disease. The FDA have termed it an inverse agonist, but this conclusion is based on in-vitro data. In this paper we discuss the evidence for such a claim being made for pimavanserin in the human brain and conclude that this is not currently sufficient. It is therefore premature to conclude that the actions of pimavanserin in humans are due to inverse agonism, and we are of the opinion that it should be called a 5-HT2A antagonist until better evidence emerges.