Antihistamines

Antihistamines are a class of agents that block histamine release from histamine-1 receptors and are mostly used to treat allergies or cold and flu symptoms, although some first-generation antihistamines may also be used for other conditions.

Histamine-1 receptors are located in the airways, blood vessels and gastrointestinal tract (stomach and esophagus). Stimulation of these receptors can lead to conditions such as a skin rash or inflammation, a narrowing of the airways (bronchoconstriction), hay fever, or motion sickness. Histamine-1 receptors are also found in the brain and spinal cord, and stimulation of these receptors makes you more awake and alert. Sedating antihistamines oppose the effects of histamine on H1 receptors in your brain, which is why they cause sedation and drowsiness.

What are antihistamines used for?

Antihistamines are very good at relieving symptoms of an allergic reaction, such as:

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

This makes antihistamines very effective in the treatment of:

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

First generation 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
• in people with Parkinson’s disease unable to tolerate more potent agents.

What are the differences between antihistamines?

Antihistamines can be classified into two main categories:

• first-generation antihistamines
• second-generation antihistamines.

First-generation 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 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 Antihistamines

Second Generation Antihistamines

What are the side effects of antihistamines?

Side effects of first-generation antihistamines may include:

• abdominal pain
• 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
• urinary problems

Side effects of second-generation antihistamines may include:

• abdominal pain
• coughing
• drowsiness
• fatigue
• a headache
• nausea
• a sore throat.