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Antihistamine is a medicine used to treat allergy. It acts by blocking the effects of histamine, a protein used by the body immune system to protect body cells from infection. The immune system is our natural body protection against diseases and infections.

If our immune system detects foreign dangerous objects such as bacteria or fungi, it will release histamine into the nearest infected cells. Histamine will then cause the dilatation of small blood vessels and swelling of the skin. This is known as inflammation.

The dilated blood vessels will cause more white blood cells to be sent to the infected areas to fight the infection. The surrounding swollen skin also helps to make it harder for the infection to spread to other body parts.

Normally, histamine is a useful protein, but sometimes its effect needs to be stopped especially when the body’s immune system mistakenly recognises harmless substances such as pollen and dust as threats.

The release of histamine will trigger inflammation and cause nearby infected tissues to become red and swollen. It will also affect the nerves inside the skin, causing itchiness.

Examples Of Allergic Condition That Can Be Treated Using Antihistamine

  • Allergic fever

  • Allergic rhinitis (inflammation of the nose due to allergic reaction to substances such as dust)

  • Allergic skin conditions such as eczema and urticaria

  • Allergic conjunctivitis (inflamed eyes)

Types Of Antihistamine

Antihistamines are classified into two groups:

  1. First generation

    Cause drowsiness for most people.

    Examples: diphenhydramine and chlorpheniramine

  2. Second generation

    Often do not cause drowsiness.

    Example: loratadine and cetirizine.

Normally, patients are recommended to use second generation antihistamines, to prevent them from experiencing side effects such as drowsiness.

For patients taking first-generation antihistamines, they are advised to be wary of the drowsiness side effect. It is advisable to avoid driving vehicles and operating machinery after taking the drug to prevent any accident.

However, this drowsiness effect can in fact help patients who have difficulty in sleeping caused by itchiness due to allergy.

Side Effects

  • Drowsiness and lethargy

  • Dizziness

  • Hypertension

  • Difficulty in concentrating

  • Stomach ache

  • Constipation

  • Blurred vision

  • Dry mouth

Patient’s Safety

Antihistamines are generally considered safe. However, there are certain health conditions that can be aggravated by taking antihistamines, or cause the antihistamine to react adversely. Therefore, patients should seek advice from their doctor or pharmacist before taking antihistamines if they have the following conditions:

  • Asthma

  • Diabetes Mellitus (Type 1 and 2)

  • Hypertension

  • Seizures

  • Glaucoma

  • Enlargement of prostate gland

  • Heart conditions

  • Renal failures

  • Bladder damage

  • Liver damage

  • Intestinal blockage

  • Hyperthyroid

  • Women who are pregnant or breast-feeding

  • Children, especially below 2 years old

Last Reviewed : 23 April 2014
Writer : Nurhazwani bt. Mohd Noor
Reviewer : Norhayati bt. Musa