beth israel deaconess medical center a harvard medical school teaching hospital

To find a doctor, call 800-667-5356 or click below:

Find a Doctor

Request an Appointment

left banner
right banner
Smaller Larger


Non-Prescription Antihistamines

Many antihistamine drugs are available without a prescription.

First-Generation Antihistamines

Examples include the first-generation antihistamines such as:

  • Brompheniramine (Dimetapp, Bromphen, Dimetane, Nasahist)
  • Chlorpheniramine (Chlor-Trimeton)
  • Clemastine (Allerhist, Tavist)
  • Diphenhydramine (Benadryl)

Second-Generation Antihistamines

Examples of second-generation antihistamine include:

  • Loratadine (Claritin) (does not cause drowsiness)
  • Cetirizine (Zyrtec)

Prescription Antihistamines


Common prescription antihistamines include:

  • Desloratadine (Clarinex)
  • Fexofenadine (Allegra)

These antihistamines (and loratadine) are less likely to cause adverse effects like drowsiness or dry mouth.

Nasal Sprays

Several antihistamine nasal sprays (for example, azelastine [Astelin]) 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. By occupying the histamine receptor sites, they prevent histamine from causing 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

How often should medication be taken?

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).

Do they interact with other drugs or food?

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.

What are the 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 doctor before taking antihistamines.

Contact Information

Allergy and Inflammation - Research
Department of Medicine
Beth Israel Deaconess Medical Center
Center for Life Science, 9th floor
330 Brookline Avenue
Boston, MA 02215