Maropitant citrate hydrate is a specific neurokinin-1 (NK-1) receptor antagonist that selectively inhibits the production of substance P, a mediator involved in signalling vomiting. Maropitant citrate was therefore originally used to treat vomiting in dogs and cats. Currently, it is also used as an adjunct to visceral analgesia in dogs and cats, and in other species such as horses and birds.