Histasin (Cetirizine Dihydrochloride)

Histasin is indicated for several common allergic conditions that affect the upper respiratory tract and skin.

• Relief of symptoms associated with seasonal or perennial allergic rhinitis, such as sneezing, nasal congestion, and itchy nose.
• Management of allergic conjunctivitis, including itchy, watery eyes caused by pollen or other environmental allergens.
• Treatment of chronic idiopathic urticaria (hives) to reduce itching and the formation of wheals.
• Adjunctive therapy for other histamine‑mediated allergic reactions when a non‑sedating antihistamine is preferred.
• Use in pediatric patients aged six years and older when a non‑sedating oral antihistamine is required, following local dosing recommendations.
• Possible off‑label use for relief of mild allergic skin reactions such as insect bites, under physician guidance.

• Cetirizine dihydrochloride acts as a selective peripheral H1‑receptor antagonist. By binding to the H1 histamine receptors on mast cells, basophils, and other target tissues, it blocks the action of endogenous histamine released during allergic responses. This inhibition prevents the intracellular signaling cascade that leads to vasodilation, increased vascular permeability, and sensory nerve activation, thereby reducing the typical signs of allergy such as itching, swelling, and rhinorrhea.

Like other antihistamines, cetirizine may cause unwanted effects, most of which are mild and reversible. These reactions are reported in clinical studies and post‑marketing surveillance.

Common

• Drowsiness or mild sedation, especially at higher doses or when combined with alcohol.
• Dry mouth, which may be alleviated by increased fluid intake.
• Headache, typically transient and resolving without intervention.

Serious

• Severe allergic reaction (anaphylaxis), presenting with difficulty breathing, swelling of the face or throat, and rapid pulse.
• Thrombocytopenia (low platelet count), which can lead to easy bruising or prolonged bleeding.
• Stevens‑Johnson syndrome, a rare but serious skin disorder characterized by blistering and detachment of the epidermis.

When considering Histasin, certain precautions should be observed to ensure safe use and to minimize the risk of adverse outcomes.

• Assess renal and hepatic function before initiating therapy, as dose adjustment may be required in impairment.
• Use caution in patients with a history of severe allergic reactions or anaphylaxis to other antihistamines.
• Avoid driving or operating machinery if drowsiness occurs, especially during the first few doses.
• Discuss pregnancy or breastfeeding status with a healthcare professional, since safety data are limited.
• Monitor for signs of blood disorders, such as unexplained bruising or prolonged bleeding, during prolonged treatment.

Cetirizine can interact with other medicines, which may alter its effectiveness or increase the likelihood of side effects.

Avoid

• Co‑administration with potent CYP3A4 inhibitors (e.g., ketoconazole, erythromycin) that may increase cetirizine plasma concentrations.
• Use with other sedating antihistamines or central nervous system depressants, which can enhance drowsiness.

Use with caution

• Concurrent use with alcohol, as it may potentiate sedation.
• Combination with other H1‑antagonists may increase the risk of anticholinergic effects.
• Use with warfarin or other anticoagulants requires monitoring of bleeding parameters.
• Concomitant use with theophylline may affect metabolism, though clinical significance is low.
• Caution when combined with monoamine oxidase inhibitors (MAOIs), as rare reports suggest possible additive central effects.