Antihistamines work by blocking the action of histamine, a chemical that is released by the immune system in response to allergens such as pollen, dust, or pet dander. Histamine can cause symptoms such as itching, sneezing, runny nose, and hives. By blocking histamine, antihistamines can reduce or prevent these symptoms. Antihistamines are commonly used to treat seasonal allergies, allergic rhinitis, and allergic skin conditions. Some antihistamines may cause drowsiness, while others are less likely to cause this side effect.