Antihistamines are a class of drugs that help treat conditions caused by too much histamine, a chemical created by your body’s immune system. They are most commonly used to treat symptoms of allergies, such as congestion, runny nose, sneezing, or itching, as well as swelling of the nasal passages, hives, and other skin rashes. Antihistamines work by blocking the effects of histamine, which is released by the immune system in response to allergens. There are two major subtypes of antihistamines:

• H-1 receptor antagonists or H-1 blockers: This subtype of antihistamines is used to treat allergy symptoms. They are available in different forms, including pills, chewable tablets, capsules, liquids, nasal sprays, and eye drops.

• H-2 receptor antagonists or H-2 blockers: This subtype of antihistamines is used to treat gastrointestinal conditions, including gastroesophageal reflux disease (also called acid reflux), peptic ulcers, gastritis, motion sickness, nausea, and vomiting.

Antihistamines can be bought over the counter or prescribed by a doctor, depending on the type and severity of the condition being treated. They come in different forms, including tablets, capsules, liquids, nasal sprays, and eyedrops. Some antihistamines cause less sleepiness than others, and some are combined with a decongestant, a drug that dries up your nasal passages. The side effects of antihistamines include dry mouth, drowsiness, dizziness, nausea and vomiting, restlessness or moodiness (in some children), trouble peeing or not being able to pee, blurred vision, and confusion. It is important to read the label carefully and ask a pharmacist or healthcare provider for advice if you are unsure which medicine to try.