Diphenhydramine is an antihistamine that has a sedative effect used in treating allergic reactions. It works by blocking the effect of histamine at h1 receptor sites and reduce the muscle contractions, thus reducing the onset of allergic symptoms. It is prescribed for allergic conditions such as seasonal allergic rhinitis, chronic urticaria, and also to treat allergic conjunctivitis, the itching and swelling of the eyes caused by seasonal allergens such as pollens and indoor allergens such as dust and dander. Diphenhydramine will take effect an hour after taking it and the effects will last for around 4-6 hours. Other than being an antihistamine, diphenhydramine also has anticholinergic (drying) effects when it is used to prevent motion sickness and to treat cases of Parkinsonism.
Also, because of its sedating properties, it is also used to treat insominia.