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.
Most of the older antihistamines have a sedative effect, and must be avoided where alertness is important. They are still widely used for night-time use. Some modern antihistamines have the opposite effect - they act as stimulants and are generally used for day-time use as they can cause insomnia.