The active ingredient of Cerenia is maropitant citrate, a neurokinin-1 (NK-1) receptor antagonist. Maropitant blocks the action of substance P- a neurotransmitter involved in vomiting—in the central nervous system. By blocking substance P from binding to the NK1 receptor, maropitant can prevent vomiting. Cerenia starts working quickly to prevent or treat your pet’s vomiting. It’s effective for about 24 hours.