Antihistamines are used to relieve or prevent the symptoms of hay fever and other allergies. They work by preventing the effects of a substance called histamine, which is produced by the body. Histamine can cause itching, sneezing, runny nose, and watery eyes. Also, in some persons histamine can close up the bronchial tubes (air passages of the lungs) and make breathing difficult. The main ingredient in antihistamines is diphenhydramine – made from petroleum products.