Cyproheptadine is an antihistamine that is commonly used in pets for the treatment of many symptoms including allergies, loss of appetite, and increased serotonin production associated with Cushing's disease. It does not treat the underlying problems causing these symptoms.
Along with it's antihistamine effects, Cyproheptadine also works as an anti-serotonin agent. Serotonin is a neurotransmitter in the brain, and when it is suppressed, it leads to an increase in appetite. This is especially useful in cats undergoing chemotherapy, as they tend to lose their appetite. Diseases such as Cushing's lead to an increase in serotonin, so cyproheptadine may be used to reduce serotonin levels.