Hay fever pill on a bubble pack, with flowers. It contains an antihistamine and is used to treat allergic rhinitis (hay fever). Antihistamines block the effect of the chemical histamine, produced by the body during an allergic reaction to the pollen produced by plants. Histamine causes redness, itching and swelling in tissues of the eyes and nose, resulting in sneezing and excess nasal secretions. Antihistamines are also used in cough remedies, as sedatives and as an emergency treatment for anaphylactic shock (a severe allergic reaction).