An allergy is an immune response, or reaction, to substances (allergens) that are usually not harmful. In someone with allergies, the immune response is oversensitive. When it recognizes an allergen, the immune system launches a response. Chemicals such as histamines are released. These chemicals cause allergy symptoms.

One type of medicine that helps relieve allergy symptoms is an antihistamine.

Allergic rhinitis - antihistamine; Hives - antihistamine; Allergic conjunctivitis - antihistamine; Urticaria - antihistamine; Dermatitis - antihistamine; Eczema - antihistamine

Antihistamines are medicines that treat allergy symptoms by blocking the effects of histamine. Antihistamines come as pills, chewable tablets, capsules, liquids, nasal sprays, and eye drops. There are also injectable forms used mainly in health care settings.

Antihistamines treat these allergy symptoms:

• Congestion, runny nose, sneezing, or itching
• Swelling of the nasal passages
• Hives and other skin rashes
• Itchy, runny eyes

Treating symptoms can help you or your child to feel better during the day and sleep better at night.

Depending on your symptoms, you can take antihistamines:

• Every day, to help keep daily symptoms under control
• Only when you have symptoms
• Before being exposed to things that often cause your allergy symptoms, such as a pet or certain plants

For many people with allergies, symptoms are the worst around 4 a.m. to 6 a.m. Taking an antihistamine at bedtime may help you or your child feel better in the morning during allergy season.

You can buy many different brands and forms of antihistamines without a prescription.

• Some work for only 4 to 6 hours, while others last for 12 to 24 hours.
• Some are combined with a decongestant, a drug that dries up your nasal passages.

Ask your health care provider what type of antihistamine and what exact dosage is right for you or your child. Make sure you understand how much to use and how many times a day to use it. Be sure to read the label carefully. Or ask your pharmacist if you have questions.

• Some antihistamines cause less sleepiness than others. These include cetirizine (Zyrtec), desloratadine (Clarinex), fexofenadine (Allegra), levocetirizine (Xyzal), and loratadine (Claritin).
• Do not drink alcohol when you are taking antihistamines.

Also, remember:

• Store antihistamines at room temperature, away from heat, direct light, and moisture.
• Do not freeze antihistamines.
• Keep all medicines where children cannot reach them.

Ask your provider if antihistamines are safe for you or your child, what side effects to watch for, and how antihistamines may affect other medicines you or your child take.

• Antihistamines are thought to be safe for adults.
• Most antihistamines are also safe for children over 2 years old.
• If you are breastfeeding or pregnant, ask your provider if antihistamines are safe for you.
• Adults who take antihistamines should know how the medicine affects them before driving or using machinery.
• If your child is taking antihistamines, make sure the medicine is not affecting your child's ability to learn.

There may be special precautions for using antihistamines if you have:

• Diabetes
• Enlarged prostate or problems passing urine
• Epilepsy
• Heart disease or high blood pressure
• Increased pressure in the eye (glaucoma)
• Overactive thyroid

Side effects of antihistamines may include:

• Changes in vision, such as blurry vision
• Decreased appetite
• Dizziness
• Drowsiness
• Dry mouth
• Feeling nervous, excited, or irritable

Call your provider if:

• Your nose is irritated, you are having nosebleeds, or you have any other new nasal symptoms
• Your allergy symptoms are not getting better
• You are having trouble taking your antihistamines

Corren J, Baroody FM, Togias A. Allergic and nonallergic rhinitis. In: Burks AW, Holgate ST, O'Hehir RE, et al, eds. Middleton's Allergy: Principles and Practice. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 40.

Seidman MD, Gurgel RK, Lin SY, et al. Clinical practice guideline: allergic rhinitis. Otolaryngol Head Neck Surg. 2015;152(1 Suppl):S1-S43. PMID: 25644617 pubmed.ncbi.nlm.nih.gov/25644617/.

Wallace DV, Dykewicz MS, Oppenheimer J, Portnoy JM, Lang DM. Pharmacologic treatment of seasonal allergic rhinitis: synopsis of guidance from the 2017 joint task force on practice parameters. Ann Intern Med. 2017;167(12):876-881. PMID: 29181536 pubmed.ncbi.nlm.nih.gov/29181536/.