ask your pharmacist

What is the difference between prescription and over-the-counter antihistamines?

Over-the-counter antihistamines include diphenhydramine (brand names Benadryl, Allerest, and Contac, doxylamine (Alka-Seltzer Cold & Sinus), clemastine (Tavist), chlorpheniramine (Chlor-Trimeton), and now loratadine (Claritin).
Antihistamines available by prescription include fexofenadine (Allegra), and cetirizine (Zyrtec).

The main difference between most over-the-counter and prescription antihistamines is in their risk of causing drowsiness. In general, prescription antihistamines cause less drowsiness--although Zyrtec may cause more drowsiness than Claritin or Allegra. Nonprescription antihistamines cause drowsiness to varying degrees, with diphenhydramine and doxylamine more likely to cause drowsiness than clemastine, aloratadine, and chlorpheniramine. Loratidine should not cause drowsiness in most people.

Since it’s hard to predict how an antihistamine will affect each person, you may have to try several before finding one that works best for you. Most people with mild allergies will do fine with an over-the-counter antihistamine. If you need to be alert--for example, to drive a car or operate heavy machinery--a prescription antihistamine is the safer choice.

Sources:
Allegra product information. Aventis Pharmaceuticals, 2006.
Claritin product information. Schering Corporation, 2006.
Drug Facts and Comparisons. Facts & Comparisons, 2006.
Handbook of Nonprescription Drugs. American Pharmaceutical Association, 2000.
Micromedex® Healthcare Series. Thomson Micromedex, 2006.
Zyrtec product information. Pfizer, 2006.

This answer prepared 7/2/2001.
This information updated 1/25/2007.