Question

Which of the following antihistamine does not affect secretion of acid? (A) Cimetidine (B) Ranitidine (C) Terfenadine (D) Histamine

Short answer

The antihistamine that does not affect the secretion of acid is Terfenadine (C). This is because it acts on a different type of histamine receptor (H1) and is mainly used for reducing allergy symptoms, not for controlling stomach acid production.

Step-by-step solution

Briefly describe each antihistamine drug

Cimetidine (A) is an H2 receptor antagonist, which blocks histamine from binding to its receptor in the stomach, and therefore reduces stomach acid secretion. Ranitidine (B) is also an H2 receptor antagonist, and it works similarly to cimetidine by reducing stomach acid secretion. Terfenadine (C) is a first-generation H1 receptor antagonist, which means it blocks histamine from binding to its receptor in the body and helps to reduce allergy symptoms. This type of antihistamine does not have any effect on acid secretion in the stomach because it acts on a different type of histamine receptor (H1, not H2). Histamine (D) is not an antihistamine drug, but rather the molecule that binds to histamine receptors and causes the release of stomach acid and allergy symptoms.

Identify the antihistamine that does not affect acid secretion

After studying each antihistamine drug, we can conclude that Terfenadine (C) is the antihistamine that does not affect the secretion of acid. This is because it acts on a different type of histamine receptor (H1) and is mainly used for reducing allergy symptoms, not for controlling stomach acid production.

Key concepts

H1 Receptor Antagonists

H1 receptor antagonists, often referred to as antihistamines, are primarily used to treat allergic reactions. These drugs work by blocking the H1 receptors, thereby preventing histamine, a chemical involved in allergic responses, from binding to them.
When histamine binds to H1 receptors, it can cause symptoms such as itching, swelling, and redness. These are common in allergies. By using H1 receptor antagonists, these symptoms can be alleviated.
Some common H1 receptor antagonists include:

• Terfenadine: A first-generation antihistamine that was widely used before being replaced by newer alternatives due to safety concerns.
• Loratadine and Cetirizine: These are second-generation antihistamines that have fewer sedative effects compared to first-generation ones.

H1 receptor antagonists are effective for allergy symptoms but do not help with stomach-related issues, since they do not influence the H2 receptors involved in stomach acid production.

H2 Receptor Antagonists

H2 receptor antagonists are drugs that reduce the production of stomach acid. They achieve this by blocking the action of histamine on the H2 receptors located on the cells lining the stomach.
By inhibiting these receptors, the release of stomach acid is decreased, providing relief from conditions like heartburn, acid reflux, and gastric ulcers. These medications are particularly useful for individuals who experience frequent or chronic acid-related disorders.
Common examples of H2 receptor antagonists include:

• Cimetidine
• Ranitidine
• Famotidine

These medications help by providing a targeted approach to decrease stomach acid production, directly addressing issues related to excess stomach acid. Unlike H1 antagonists, they are not effective for allergies, as they act on a different receptor in the body.

Stomach Acid Secretion

Stomach acid secretion is a natural process that is crucial for digesting food and defending against pathogens. This process is stimulated by various signals, including the presence of food and the release of certain molecules like histamine.
Histamine plays a significant role in regulating stomach acid by binding to H2 receptors on the stomach lining. When it binds, it triggers the cells to release more acid, aiding digestion.
However, excessive stomach acid can lead to discomfort and conditions such as:

• Heartburn
• Acid reflux
• Peptic ulcers

H2 receptor antagonists are often used to manage these conditions by interfering with histamine's ability to stimulate acid production. This targeted intervention helps maintain a balance by reining in excess acid while allowing necessary digestive functions to continue.