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Antihistamines block histamine release from histamine-1 receptors and are mostly used to treat allergies or cold and flu symptoms, although some first-generation antihistamines (also called sedating antihistamines) may be used for other conditions, such as nausea or motion sickness.

Histamine-1 receptors are located in your airways (breathing tubes), blood vessels, stomach, and esophagus (throat area). Allergens, such as pollen or pet fur can stimulate these receptors, causing the release of histamine, which results in symptoms such as a rash, sneezing, a runny nose, or a narrowing of the airways (also called bronchoconstriction). Histamine-1 receptors are also found in the brain and spinal cord and first-generation antihistamines (sedating antihistamines) can cross through the blood/brain barrier into the brain and act on these receptors, causing sleepiness (sedation), and drowsiness.

What are antihistamines used for?

Antihistamines can help relieve the symptoms of an allergic reaction, such as:

• a rash
• itchiness
• redness (inflammation)
• swelling (edema)
• red and watery eyes
• a runny nose
• sneezing.

This makes antihistamines very effective for the treatment of:

• allergic rhinitis
• colds
• flu
• food allergies
• hay fever
• hives (urticaria)
• hypersensitivity reactions to drugs
• insect bites
• stings.

Sedating antihistamines (see explanation below) also act in the brain and spinal cord, and on other receptors. This makes some of them also useful for:

• inducing sleep
• preventing or treating motion sickness
• reducing anxiety
• some symptoms of Parkinson’s disease in people unable to tolerate traditional medications for Parkinson's disease.

What are the differences between antihistamines?

Antihistamines can be classified into two main categories:

• first-generation antihistamines (also known as sedating antihistamines because they all cause drowsiness)
• second-generation antihistamines (also known as non-sedating antihistamines because they don't tend to cause drowsiness, although they may do so in some people).

First-generation (sedating) antihistamines were developed more than seventy years ago and are still in widespread use today. They act on histamine receptors in the brain and spinal cord and in the rest of the body (called the periphery). They also act on muscarinic, alpha-adrenergic, and serotonin receptors. This means that first-generation antihistamines are more likely to cause side effects such as sedation, dry mouth, dizziness, low blood pressure, and a rapid heart-beat. They are also more likely than second-generation antihistamines to impair a person’s ability to drive or operate machinery. Interactions with other drugs are more common with first-generation antihistamines compared with second-generation antihistamines.

Second generation (non-sedating) antihistamines were developed in the 1980s and are much less sedating than first-generation antihistamines. They act on histamine-1 receptors in the periphery and are unlikely to penetrate the brain, so are less likely to cause side effects or interact with drugs. Most second-generation antihistamines do not cause drowsiness, although some (such as cetirizine and fexofenadine), may be more likely to do so at higher dosages.

Common antihistamines available in the U.S.

First Generation (Sedating) Antihistamines

Second Generation (Non-sedating) Antihistamines

What are the side effects of antihistamines?

Side effects of first-generation (sedating) antihistamines may include:

• blurred or double vision
• constipation
• dry eyes
• a dry mouth
• drowsiness or sedation
• a headache
• low blood pressure
• mucus thickening in the airways
• a rapid heartbeat
• stomach pain
• urinary problems

Side effects of second-generation (non-sedating) antihistamines may include:

• coughing
• drowsiness (this is uncommon)
• fatigue 
• a headache
• nausea
• a sore throat
• stomach pain.