Overview
Cetirizine is a widely used, second-generation antihistamine. It is a major metabolite of the first-generation drug hydroxyzine but has been chemically modified to be significantly less able to cross the blood-brain barrier. It is highly effective for the symptomatic treatment of seasonal and perennial allergic rhinitis (hay fever), as well as chronic idiopathic urticaria (hives), providing long-lasting relief from itching, sneezing, and runny nose.

Mechanism of Action
Cetirizine is a potent and highly selective antagonist of the peripheral H1 histamine receptors. By firmly binding to these receptors on cells throughout the body (excluding the brain), it blocks the action of histamine released during allergic responses. This prevents histamine from triggering the cascade of inflammatory symptoms, effectively reducing vascular permeability, itching, and smooth muscle constriction.

Dosage and Administration
The standard adult dosage is typically 10 mg taken once daily. For some patients, particularly those who may be sensitive to its effects or for older adults, a lower dose of 5 mg daily may be sufficient. It has a rapid onset of action, often providing relief within one hour, and its effects last for a full 24 hours. It can be taken with or without food.

Side Effects
As a second-generation antihistamine, cetirizine is specifically designed to be ‘non-drowsy’ or minimally sedating compared to older drugs. However, among the second-generation options, it is slightly more likely to cause mild sedation in a small percentage of users. Other potential, though uncommon, side effects include dry mouth, headache, fatigue, and mild gastrointestinal upset.

Contraindications and Precautions
Cetirizine is contraindicated in patients with a known hypersensitivity to it or to hydroxyzine. It should be used with caution and often at a reduced dosage in patients with moderate to severe renal impairment, as the drug is primarily excreted unchanged by the kidneys. While generally non-sedating, patients should still observe how they react before driving or operating machinery.

Pregnancy and Lactation
Cetirizine is generally considered safe for use during pregnancy and is often a preferred antihistamine when treatment is necessary. However, it is excreted in human breast milk. While occasionally used during lactation, it is generally advised to exercise caution or consider alternatives, as it may cause mild sedation in the nursing infant.