Antihistamines for Treating Allergies

Antihistamines come in tablet, capsule, liquid, injectable and nasal spray forms. They're available over the counter (OTC) or by prescription. Because antihistamines don't help stuffy noses, they're often sold in combination with decongestants.

When You Take Antihistamines to Treat Allergies

  • They work by preventing histamines from attaching to certain sites on cells and causing allergic reactions and allergy symptoms.
  • Some over-the-counter antihistamines and some prescription antihistamines can cause drowsiness. Be sure you know when it's safe to use them.
  • Don't drink alcohol while taking antihistamines.
  • Don't take antihistamines without your doctor's approval if you're pregnant, trying to get pregnant, or nursing.

For more than 50 years, antihistamines have been the first type of allergy medication chosen to treat nasal allergies. They are a great way to head off allergy symptoms before they start. They are especially effective against runny noses, sneezing, and itching. However, they do little to relieve nasal congestion.


Many antihistamines are available without a prescription, and some are quite inexpensive. Some can cause drowsiness or sedation, however. There are newer antihistamines available that don't cause as much drowsiness. These include the the prescription drug Allegra (fexofenadine), and Zyrtec (cetirizine) and Claritin (loratadine), which are now available without a prescription.

Sedating versus Nonsedating Antihistamines to Treat Allergies

Sedating Antihistamines. You may have taken antihistamines in the past and found it hard to concentrate or stay awake. For many people, sedating antihistamines, also called old, classic, or first-generation antihistamines, cause sleepiness, grogginess, and slow reaction time. They may interfere with coordination and cloud your concentration.

Many sedating antihistamines do not require a doctor's prescription. Sold over the counter (OTC), they are less expensive than prescription nonsedating antihistamines.

Nonsedating Antihistamines. Nonsedating antihistamines, also called new, or second-generation antihistamines, are just as effective against nasal allergy symptoms as older medications. They simply do the job with fewer side effects. They make you less sleepy and groggy, and are less likely to cause problems with increased eye pressure, which may worsen glaucoma symptoms.

Another advantage of the newer antihistamines is that they're available in time-release versions. That means you can control your allergy symptoms with only 1 or 2 doses each day compared with older medications, which usually require doses every 4 to 6 hours to maintain their effectiveness.

Some newer antihistamines are available only by prescription, and some are available over-the-counter. Your doctor can help you make the right choice. Because some can cause serious side effects or interact with other medications you are taking, be sure to let your doctor know all the medications you take.


Using Antihistamines Safely

Discuss with your doctor the side effects of any medicine you take. With antihistamines, even those bought over the counter, follow these tips to be sure you use them safely.

  • Watch for drowsiness. Feeling sleepy and "medicated" is common with sedating antihistamines, but you may not even be aware that you are sluggish.
  • Don't drive, operate machinery, or perform other dangerous tasks if the antihistamine makes you drowsy. Some people who take antihistamines don't feel impaired by the medications. But they really are! That's why you need to follow the directions on the box or those provided by your pharmacist or doctor.
  • Don't drink alcohol while taking antihistamines. Alcohol is a depressant that can increase the sedating or drowsy effects of antihistamines. Taking certain kinds of painkillers while taking antihistamines can make you even less alert and more drowsy. Check with your doctor or pharmacist if you have a question about mixing medications.
  • Try taking sedating antihistamines at bedtime. That way, any drowsiness will cause fewer problems. Be aware, though, that even if you take it at night, the antihistamine may still cause some impairment the next day.
  • Report any side effects to your healthcare provider.
  • Don't take antihistamines without your doctor's approval if you're pregnant, trying to get pregnant or nursing.
  • Avoid dangerous drug interactions. Make sure your doctor and pharmacist are aware of all medicines and supplements you take.

How Antihistamines Work In Treating Allergies

Antihistamines work by blocking the effects of histamine, the chemical responsible for many of the allergy symptoms you experience. When histamine is released, it binds to special sites called receptors on cells in your nose and throat, causing them to swell and leak fluid. This results in inflammation, nasal congestion, runny nose, sneezing, itching, and other allergy symptoms.


Antihistamines block the effects of histamine by "coating" receptors, which prevents binding. This, in turn, prevents nasal allergy symptoms.

Histamine works quickly once released. By the time your allergy symptoms appear, the histamine has already attached to cell receptors, and the allergic reaction is well under way. That's why you need to take antihistamines 2 to 5 hours before exposure to allergens. Or you need to take them on a regular basis.

Some antihistamines go to work 15 to 30 minutes after they're taken. They reach their peak effectiveness in 1 to 2 hours. If you take an antihistamine before you're exposed to your allergens, the allergic reaction can be stopped.

Side Effects of Antihistamines

If you're bothered by side effects, your doctor can often help by changing:

  • How much medication you take. Sometimes side effects can be stopped or minimized by reducing the dose. Or, your doctor may lower the dose and then raise it more slowly.
  • When you take the medication. You may be able to cope with drowsiness or insomnia, for instance, by taking your medication in the evening or first thing in the morning.
  • How you take the medication. Taking your medication in smaller doses several times a day rather than in one dose can help. Taking it with food might eliminate side effects such as nausea.
  • The type of allergy medication. A different allergy medication may stop your symptoms with fewer or less severe side effects. Always talk with your doctor before changing how you take allergy or other medications.

For more information about allergy treatments, see the next page.


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Written by Karen Serrano, MD Emergency Medicine resident at the University of Wisconsin-Madison.

Reviewed by Lisa V. Suffian, MD

Instructor of Clinical Pediatrics in the Division of Allergy and Pulmonary Medicine at Saint Louis Children's Hospital, Washington University School of Medicine

Assistant Clinical Professor in the Department of Pediatrics at Cardinal Glennon Children's Hospital, Saint Louis University

Board certified in Allergy and Immunology

Last updated June 2008