**Question:** All of the statement about H1 anti histaminics are true except -

A. They are agonists of histamine receptors
B. They are antagonists of H2 histamine receptors
C. They have a direct effect on the cardiovascular system
D. They cause muscle relaxation

**Core Concept:** H1 antihistamines are a class of medications used to treat allergic reactions by blocking histamine H1 receptors. Histamine is a chemical released during an allergic response, causing symptoms such as itching, sneezing, and inflammation. H1 antihistamines block the actions of histamine on H1 receptors, reducing these symptoms.

**Why the Correct Answer is Right:** Option D (muscle relaxation) is incorrect because H1 antihistamines primarily act on peripheral H1 receptors and do not have a direct effect on the central nervous system or smooth muscle contraction. While they may indirectly affect muscle relaxation by reducing inflammatory mediators, the primary mechanism is through blocking H1 receptors in the peripheral tissues.

**Why Each Wrong Option is Incorrect:**

A. **They are agonists of histamine receptors:** H1 antihistamines are antagonists (blockers), not agonists (activators) of histamine receptors. They bind to H1 receptors, preventing histamine from binding and activating them, thus reducing allergic symptoms.

B. **They are antagonists of H2 histamine receptors:** While some H1 antihistamines may also act as H2 antagonists, the primary action is on H1 receptors. This is why the correct answer is not "They are antagonists of H2 histamine receptors."

C. **They have a direct effect on the cardiovascular system:** H1 antihistamines primarily affect histamine on the peripheral tissues, not directly on the cardiovascular system. Their influence on the cardiovascular system is indirect, as they reduce histamine-induced vasodilation and increased vascular permeability, which can lead to hypotension in some patients.

**Clinical Pearl:** It is essential for students to understand the primary mode of action of H1 antihistamines and not confuse their effects with other medications or physiological processes. While some H1 antihistamines may have additional effects, the primary action lies in blocking H1 receptors, which reduces allergic symptoms.