Cetirizine

What is Cetirizine?

Cetirizine is a second-generation antihistamine used to relieve symptoms associated with allergic rhinitis, seasonal allergies, and chronic urticaria (hives).

Compared with first-generation antihistamines, cetirizine produces significantly less sedation because its molecular structure limits penetration across the blood–brain barrier.

History & Development

Cetirizine is an active metabolite of hydroxyzine and was developed to preserve strong anti-allergic properties while minimizing central nervous system depression. Its improved safety profile contributed to its widespread global use.

Chemical Structure & Medicinal Chemistry

• IUPAC: (±)-2-[4-[(4-chlorophenyl)phenylmethyl]-1-piperazinyl]ethoxyacetic acid
• Molecular Formula: C₂₁H₂₅ClN₂O₃
• Molar Mass: 388.89 g/mol
• Key Functional Groups: Piperazine ring, carboxylic acid, aromatic rings

Its polarity reduces central nervous system penetration, which explains the lower incidence of drowsiness compared to older antihistamines.

Mechanism of Action

Cetirizine selectively blocks peripheral histamine H₁ receptors. Histamine release during allergic reactions normally causes vasodilation, increased vascular permeability, and sensory nerve stimulation.

By preventing histamine from binding to these receptors, cetirizine reduces symptoms such as sneezing, itching, rhinorrhea, and watery eyes.

Pharmacokinetics

• Rapid oral absorption
• Peak plasma concentration ~1 hour
• Protein binding ~93%
• Minimal hepatic metabolism
• Primarily excreted unchanged via kidneys

Clinical Uses

• Allergic rhinitis (hay fever)
• Seasonal allergies
• Chronic urticaria
• Dust and pollen allergies
• Symptomatic relief of itching

Safety and Responsible Use

Cetirizine is generally well tolerated when used at recommended doses. However, patients should remain aware of potential mild sedation, especially when combining it with other central depressants.

Common Side Effects

• Mild drowsiness
• Dry mouth
• Headache
• Fatigue

Who Should Use Caution?

• Patients with kidney impairment (dose adjustment may be required)
• Individuals taking sedative medications
• Older adults sensitive to antihistamines

Drug Interactions

• Alcohol — may enhance sedation
• Sedatives — additive CNS depression
• Other antihistamines — increased side-effect risk

Cetirizine vs Loratadine

Cetirizine often demonstrates a faster onset of action, while loratadine is typically associated with even less sedation. Therapeutic choice depends on patient sensitivity and clinical preference.

References

• World Health Organization (WHO)
• U.S. Food and Drug Administration (FDA)
• PubChem Drug Database
• National Center for Biotechnology Information (NCBI)