Antihistamine

1.0What is an Antihistamine?

An antihistamine is a type of drug that helps reduce or block the effects of histamine — a chemical produced by the body during an allergic reaction. Histamine binds to receptors in the body, causing symptoms like itching, sneezing, watery eyes, and runny nose.

By blocking histamine receptors, antihistamines help relieve these allergic symptoms, making them a common medication for treating allergies, colds, and other hypersensitivity reactions.

2.0Understanding Histamine and Its Role in the Body

Histamine is a biogenic amine naturally present in our body. It is stored in certain white blood cells, such as mast cells and basophils.

When the immune system detects an allergen — like pollen, dust, or certain foods — it releases histamine. This chemical then:

• Expands blood vessels (vasodilation)
• Increases permeability of capillaries
• Causes inflammation, itching, and redness

Antihistamines act as histamine antagonists, preventing this chain reaction by binding to histamine receptors before histamine can.

3.0Mechanism of Action of Antihistamines

Antihistamines primarily work by blocking the H1 and H2 histamine receptors located throughout the body.

H1-Receptor Antagonists

• Target the H1 receptors found in smooth muscles, endothelium, and the central nervous system.
• They prevent allergic responses like sneezing, nasal congestion, and skin irritation.
• Commonly used in conditions such as allergic rhinitis, urticaria, and conjunctivitis.

H2-Receptor Antagonists

• Act on H2 receptors present in the stomach lining.
• They reduce the secretion of gastric acid, helping treat peptic ulcers and acid reflux.
• Examples include ranitidine and famotidine.

4.0Classification of Antihistamines

Antihistamines are categorized into generations based on their CNS penetration (sedative potential) and receptor selectivity.

1. First-Generation Antihistamines (Sedating)

These are the oldest class. They are lipophilic (fat-loving), allowing them to easily cross the Blood-Brain Barrier (BBB).

Characteristics:

• Low Selectivity: They bind to H1​ receptors but also block cholinergic (muscarinic), alpha-adrenergic, and serotonin receptors.
• Sedation: High affinity for H1​ receptors in the brain causes drowsiness.
• Short Duration: usually require multiple doses per day.

Common Examples:

• Diphenhydramine (Benadryl)
• Chlorpheniramine
• Promethazine
• Hydroxyzine

2. Second-Generation Antihistamines (Non-Sedating)

Developed to maximize allergic relief while minimizing sedation. They are lipophobic (large, polar molecules) and do not readily cross the BBB.

Characteristics:

• High Selectivity: Highly specific for peripheral H1​ receptors; little effect on muscarinic receptors.
• Non-Sedating: Limited CNS entry means they do not cause significant drowsiness at standard doses.
• Long Duration: Often dosed once daily (24 hours).

Common Examples:

• Loratadine (Claritin)
• Cetirizine (Zyrtec)
• Fexofenadine (Allegra)

3. Third-Generation Antihistamines

These are active enantiomers or metabolites of second-generation drugs, designed for increased potency and fewer side effects.

Examples:

• Levocetirizine: The active enantiomer of Cetirizine.
• Desloratadine: The active metabolite of Loratadine.

5.0Uses of Antihistamines in Medicine

Antihistamines are prescribed for a variety of conditions. Here are some key applications:

1. Allergy Relief

Used to treat symptoms of seasonal allergies (hay fever), such as:

• Sneezing
• Runny nose
• Watery or itchy eyes

2. Skin Conditions

Help reduce itching and rashes caused by:

• Urticaria (hives)
• Eczema
• Insect bites

3. Motion Sickness and Nausea

Some first-generation antihistamines (like dimenhydrinate) prevent nausea and vomiting caused by motion sickness.

4. Cold and Flu Relief

Antihistamines are commonly included in over-the-counter cold medicines to reduce symptoms like sneezing and watery eyes.

5. Gastric Disorders

H2 blockers like ranitidine and famotidine are used to treat stomach ulcers and acid reflux by reducing acid production.

6.0Common Antihistamine Drugs and Brand Names

7.0Side Effects of Antihistamines

Although generally safe when used properly, antihistamines can cause side effects, especially in higher doses.

Common Side Effects

• Drowsiness (more in first-generation)
• Dry mouth and throat
• Blurred vision
• Dizziness
• Constipation or urinary retention

Less Common but Serious Effects

• Rapid heartbeat (tachycardia)
• Confusion in elderly patients
• Allergic reactions in rare cases

Note for Students: Always take antihistamines under medical supervision and avoid combining them with alcohol or sedatives.

8.0Precautions While Taking Antihistamines

1. Avoid driving or operating machinery after taking sedating antihistamines.
2. Consult a doctor before use if you have liver or kidney disease.
3. Pregnant or breastfeeding women should seek medical advice before taking antihistamines.
4. Do not mix with other CNS depressants like sleeping pills.