Antihistamines or histamine antagonists are a group of pharmaceuticals that prevent histamines from binding to their receptors in cells. Most antihistamines also produce the opposite effect from histamines, actually reducing swelling and inflammation. They are useful in preventing and counteracting the most common symptoms of an allergy, such as runny nose, nasal congestion, sneezing, excess production of tears, and skin irritation. In addition, they can be used to prevent excess stomach acid production.