Antihitamines

Introduction
Illustrated mechanism of action
Examples of Medication Brand Names (with Images)
Indication
Side Effects
Precautions & Contraindications
Monographs
Reference

History

History of Antihistamines

1937: Daniel Bovet synthesized the first antihistamine.

1940s: H1 antihistamines (e.g., mepyramine) introduced to treat allergies.

1950s–60s: Newer H1 drugs developed to reduce sedation.

1970s: Discovery of H2 receptors → led to H2 blockers like cimetidine for stomach acid issues.

1990s–present: Modern non-sedating H1 antihistamines (e.g., loratadine, cetirizine) became common for long-term allergy treatment.

Introduction

First-generation antihistamines were introduced in the 1940s, with phenbenzamine being the first clinically useful one in 1942. These drugs act as H1 receptor antagonists, blocking histamine effects responsible for allergy symptoms such as itching, hives, and runny nose. They cross the blood-brain barrier, which often causes sedation and drowsiness. Examples include diphenhydramine (Benadryl) and chlorpheniramine. Despite their side effects like dry mouth, dizziness, and sedation, they are still used today, especially when sedation might be beneficial, such as in severe itching at night.

Second-generation antihistamines were developed later to reduce sedation and other side effects by minimally crossing the blood-brain barrier. These are preferred for long-term allergy management due to better safety and fewer drug interactions.

Illustrated mechanism of action

1. Antihistamines block the effects of histamine.

• Histamine is a chemical released by the body during allergic reactions.

• It binds to histamine receptors and causes symptoms like:

• Sneezing

• Itching

• Runny nose

• Skin rashes

2. Types of Histamine Receptors:

• H1 receptors: found in the nose, skin, lungs, and brain.

• H2 receptors: found in the stomach lining (related to acid secretion).

H1 Antihistamines (like loratadine, cetirizine, diphenhydramine):

• They block H1 receptors, preventing histamine from binding.

• This reduces allergy symptoms like sneezing, itching, and rashes.

H2 Antihistamines (like ranitidine, famotidine):

• They block H2 receptors in the stomach.

• This reduces stomach acid and helps treat ulcers or acid reflux.

Examples of Medication Brand Names

Loratadine

Telfast

Levohistam

Indication

1. Allergic Conditions:

• Allergic rhinitis (e.g., hay fever)

• Urticaria (hives)

• Angioedema (swelling under the skin due to allergy)

• Atopic dermatitis (eczema)

• Allergic conjunctivitis (eye allergies)

2. Insect Bites and Stings:

• To relieve itching, redness, and swelling.

3. Common Cold and Flu:

• Often included in combination products to relieve symptoms like sneezing and runny nose (especially first-generation antihistamines).

4. Motion Sickness and Nausea:

• First-generation antihistamines like meclizine or dimenhydrinate are used to prevent or treat nausea, vomiting, and dizziness.

5. Insomnia (Sleep Aid):

• Some first-generation antihistamines (e.g., diphenhydramine) are used for short-term treatment of sleep problems due to their sedative effects.

6. Anaphylaxis (Severe Allergic Reactions):

• Used as an adjunct to epinephrine in emergency settings.

Side Effects

Side effects of first-generation H1 (allergy) antihistamines can include:

Drowsiness (sleepiness) and reduced coordination, reaction speed and judgment.

Dry mouth and dry eyes.

Blurred or double vision.

Dizziness.

Headaches.

Difficulty peeing and constipation.

Low blood pressure.

Mucous thickening in your airways.

Side effects of second-generation H1 (allergy) antihistamines can include:

Headaches.

Cough.

Sore throat.

Abdominal pain or discomfort.

Nausea or vomiting.

Drowsiness (in high doses).

Side effects of H2 (digestive) antihistamines are uncommon but can include:

Fatigue.

Diarrhea or constipation.

Joint or muscle pain.

Headache.

Confusion in people over the age of 65.

Dizziness.

Enlarged male breast tissue (gynecomastia).

Milky nipple discharge (galactorrhea).

Precautions & Contraindications

Precautions for Antihistamines:

1. Elderly:

First-generation antihistamines may cause drowsiness and dizziness, increasing the risk of falls.

2. Pregnancy and Breastfeeding:

Antihistamines should be used cautiously and under medical supervision during pregnancy and lactation, as some types may affect the fetus or infant.

3. Liver or Kidney Disease:

Since the body metabolizes or eliminates the drug through the liver or kidneys, dosage adjustment may be necessary in cases of impairment.

4. Asthma or COPD:

First-generation antihistamines can thicken respiratory secretions, making breathing more difficult in these conditions.

5. Use with CNS Depressants (e.g., alcohol, sedatives):

Concurrent use may enhance sedative effects, leading to excessive drowsiness.

------------

Contraindications:

1. Known allergy to any component of the drug:

Antihistamines must not be used in patients with known hypersensitivity.

2. Narrow-angle glaucoma:

This condition may worsen with first-generation antihistamines.

3. Urinary retention or prostate enlargement (BPH):

These drugs may make urination more difficult, worsening the condition.

4. Use in newborns or premature infants:

First-generation antihistamines are contraindicated due to risk of respiratory depression or excessive sedation.

Monograph

Drug monograph has.pdf

Drug monograph.docx_20250424_173329_٠٠٠٠.pdf

Drug monograph.docx_20250323_210846_٠٠٠٠.pdf

2664924c-52c7-4bbd-8695-ca31134298dftmp_250323_194813-٢.pdf

Document from Bassant Ahmed.pdf

Reference