An antihistamine is a type of medication that helps relieve allergy symptoms by blocking the action of histamine, a substance in the body that causes allergic reactions. Common symptoms treated by antihistamines include sneezing, runny nose, and itchy eyes. They are often used for conditions like hay fever and hives. Antihistamines come in two main types: first-generation and second-generation. First-generation antihistamines can cause drowsiness, while second-generation ones are less likely to do so, making them a popular choice for daytime use. These medications can be found in various forms, including tablets, liquids, and nasal sprays.