Drug Mechanism: How does Hydroxyzine work?

Histamine is a chemical produced by the body. It is stored in cells (mast cells) in almost all tissues of the body. When the body reacts to a foreign substance (an allergen e.g. flower pollen), the mast cells release their stores of histamine.

Histamine may then bind to receptors (H-1 receptors) in many areas of the body causing an increase in blood flow to the area of the allergy and the release of other chemicals that add to the allergic response. This stimulates the symptoms of an allergic reaction such as inflammation of the skin, airways or tissues, rashes, itching of the skin, eyes or nose, nasal congestion or narrowing of the airways.

Hydroxyzine acts by blocking the binding of histamine to receptors in the body and so preventing or relieving the typical symptoms of an allergic reaction.

Histamine may be released from and act in a small (localised) area of the body e.g. the nose. Alternatively, histamine can result in a serious or complete body reaction e.g. anaphylactic reaction. Hydroxyzine is only useful for the prevention or relief of localised or less serious allergies such as an itchy rash (rather than very serious allergic reactions).

Hydroxyzine is called a sedating anti-histamine as it enters the brain in significant quantities and is therefore more likely to cause drowsiness than the newer group of non-sedating anti-histamines.