Antihistamines for Treating Allergies

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