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Medical Author:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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Many antihistamine drugs are available without a prescription. Examples include the first-generation antihistamines such as brompheniramine (Dimetapp, Bromphen, Dimetane, Nasahist), chlorpheniramine (Chlor-Trimeton), clemastine (Allerhist, Tavist), and diphenhydramine (Benadryl), and the second-generation antihistamine loratadine (Claritin) and cetirizine (Zyrtec). Loratadine and cetirizine do not cause drowsiness.

Common prescription antihistamines (all second-generation antihistamines) include desloratadine (Clarinex), and fexofenadine (Allegra). These antihistamines (along with loratadine and cetirizine) are less likely to cause adverse effects like drowsiness or dry mouth.

Antihistamine nasal sprays, such as azelastine (Astelin) and olopatadine (Patanase), are also available to treat symptoms such as runny nose, sneezing, and itchy nose.

• How antihistamines work: These drugs compete with histamine for histamine receptor sites. Histamine is a chemical released by specialized allergy cells called activated mast cells during the allergic response. By occupying the histamine receptor sites, they prevent histamine from causing the characteristic allergic symptoms. Antihistamines are most effective when taken continuously during the allergy season.



• Who should not use these medications: Antihistamines are contraindicated in individuals allergic to them. They may cause unwanted side effects in the following:



• Individuals currently using a monoamine oxidase inhibitor (MAOI)



• Individuals with narrow-angle glaucoma



• Individuals who are breastfeeding



• Use: Antihistamines come in tablet, chewable tablet, capsule, and liquid forms. How often an antihistamine should be taken each day depends on the individual antihistamine's characteristics and the type of preparation (that is, the dosage form).



• Drug or food interactions: Avoid taking other drugs that cause drowsiness, such as alcohol, sleep preparations, sedatives, or tranquilizers. Avoid taking MAOIs (for example, isocarboxazid [Marplan], phenelzine sulfate [Nardil], or tranylcypromine [Parnate]) within 14 days of antihistamines. For drug interactions specific to a particular antihistamine, talk with a doctor or pharmacist.



• Side effects: Many antihistamines (particularly the first-generation agents) may cause the following side effects:



• Drowsiness



• Dry mouth



• Urine retention



• Blurred vision



• Before driving a car or operating machinery, be sure to know if the antihistamine affects the ability to concentrate and stay awake.



• Check with a health-care provider before taking antihistamines if you are unsure whether or not antihistamines will be appropriate for you.

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