Antihistamines are one of the most important medications for treating allergies. Antihistamines help relieve, sneezing, itching, runny nose and may help reduce congestion. Your doctor can prescribe a “non-sedating” antihistamine to help manage your symptoms without drowsiness. Many over-the-counter antihistamines cause drowsiness and carry warnings about mental alertness, especially while driving or operating machinery.

How do antihistamines work?

Antihistamines help relieve symptoms by blocking the action of histamine. Histamine is a naturally occurring substance that is released by the immune system after being exposed to an allergen. When you inhale an allergen, mast cells located in the nose and sinus membranes release histamine. Histamine then attaches to receptors on nearby blood vessels, causing them to enlarge (dilate). Histamine also binds to other receptors located in nasal tissues, causing redness, swelling, itching, and changes in the secretions. By blocking histamine receptors, antihistamines prevent symptoms.


Decongestants are available in nasal spray or pill form. Decongestants in pill (oral) form generally take longer to work, whereas nasal sprays tend to provide more immediate relief of symptoms. The pills are recommended for longer use, because decongestant nasal sprays used more than 5 to 7 days may actually worsen symptoms.

If you are taking medication to lower blood pressure, ask your doctor about taking decongestants, as they may raise blood pressure. Decongestants may also cause insomnia, irritability, or restrict urinary flow. Decongestants are not recommended for people with glaucoma.

How do decongestants work?

Nasal tissues swell in response to allergen exposure, causing increased fluid and mucus. Decongestants shrink swollen nasal tissues to relieve symptoms of nasal swelling, congestion, and mucus secretion.

Combination medications

Some allergy medications contain both an antihistamine and a decongestant to relieve multiple symptoms, including itching, sneezing, and a runny nose. Combination medications also help clear the nasal passages. Combination medications are available over-the-counter and by prescription. The prescription combination medications are less likely to cause drowsiness.

Nasal sprays

Nasal sprays help prevent nasal allergy symptoms. Nasal sprays are available in these forms:

  • Decongestant (available over-the-counter and by prescription) to clear the nasal passages. They should not be used for more than 3 days, depending on the medication, to prevent worsening of symptoms (called “rebound effect”).
  • Salt-water solution (over-the-counter) to relieve mild congestion, loosen mucus, and prevent crusting. They contain no medication.
  • Corticosteroid (available only by prescription) to reduce inflammation and other nasal allergy or non-allergic symptoms. They may not take full effect until 1 to 2 weeks of treatment.
  • Antihistamine (available by prescription). There is currently one topical antihistamine nasal spray that reduces allergic or non-allergic nasal symptoms.


Immunotherapy, or allergy shots, may be the most effective form of treatment if other medical management techniques have failed. Immunotherapy is currently indicated for people who have allergic rhinitis, allergic asthma, and bee sting or fire ant allergies.


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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition. This document was last reviewed on: 3/20/2012…#8612