Antihistamines for treating allergy symptoms: How do they work?

Antihistamines are medicines used to treat allergy symptoms. Allergies occur when your body mistakes a harmless substance for a threat and releases histamine, an immune system chemical, for protection. The effects of histamine can make you sneeze, cause your nose to run and make your eyes red, watery and itchy. Histamine can also cause you to develop a swollen, itchy rash on your skin, called hives. Antihistamines work to lessen these common allergy symptoms by blocking the histamines in your body.

An antihistamine is a type of medicine that reduces or blocks the effects of histamine produced by your body during an allergic reaction. Antihistamines work well to relieve or prevent most symptoms of seasonal and indoor allergies. However, they aren't very effective for easing nasal congestion or stuffiness. Your healthcare provider may recommend a decongestant to treat nasal congestion. Some over-the-counter (OTC) allergy medicines combine an antihistamine with a decongestant.

Antihistamines come in many forms, including liquids, pills, nasal sprays and eye drops. Different antihistamines are available OTC or by prescription.

Liquids and pills

Oral antihistamines (taken by mouth) help to relieve allergy symptoms such as a runny nose, sneezing, itchy or watery eyes, and redness or itching due to hives. Some antihistamines can cause drowsiness, so it's best to take them before bedtime. Do not use these medicines before you drive or do other activities that require you to be alert.

OTC antihistamines that tend to cause drowsiness include:

• Diphenhydramine (Benadryl)
• Chlorpheniramine (Chlor-Trimeton)

These commonly used, newer oral antihistamines generally cause less drowsiness:

• Cetirizine (Zyrtec)
• Fexofenadine (Allegra)
• Levocetirizine (Xyzal)
• Loratadine (Alavert, Claritin)
• Desloratadine (Clarinex) - requires prescription

Nasal sprays

Antihistamine sprays target nasal allergy symptoms, such as postnasal drip, congestion and sneezing. Side effects of antihistamine nasal sprays may include a bitter taste in the mouth immediately after use or drowsiness. Nasal antihistamines are available by prescription. Examples include:

• Azelastine (Astepro)
• Olopatadine (Patanase)

Eye drops

Antihistamine eye drops reduce the itching, redness and swelling associated with eye allergies. Some eye drops have a combination of antihistamines and other medicines.

The most common side effects with these medicines are headache and dry eyes. Some examples of antihistamine eye drops include:

• Azelastine (Optivar) – available by prescription
• Cetirizine (Zerviate) – available by prescription
• Ketotifen (Alaway, Zaditor)
• Olopatadine (Pataday)

Read the directions on the label before taking antihistamine drugs. Make sure you know how and when to take your medicine. Common allergy medicines may interact with other medicines you take. Your pharmacist can help make sure that all of your medicines are safe to take together. Talk to your healthcare provider before using OTC allergy medicines, especially if you are pregnant, nursing or have a chronic disease or condition, like heart disease or high blood pressure.

Clinically reviewed and updated September 2023.