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Second-generation H1 antihistamine

Cetirizine

Cetirizine is a second-generation H1 receptor antagonist widely used for allergic rhinitis and chronic urticaria. Unlike first-generation antihistamines, it has minimal CNS penetration and reduced sedation. The most testable fact is that cetirizine is a carboxylic acid metabolite of hydroxyzine with significantly less anticholinergic activity.

Practice questionsHigh-yield facts
Drug classSecond-generation H1 antihistamine
MechanismSelective peripheral H1 receptor antagonist
Main usesAllergic rhinitis, chronic idiopathic urticaria
Key adverse effectSedation (dose-dependent, less than first-generation)
5 min readFor medical studentsUpdated 2026-10-15

Mechanism of action

Cetirizine is a selective inverse agonist at peripheral histamine H1 receptors. By blocking H1 receptors on vascular endothelium, smooth muscle, and sensory nerve endings, cetirizine prevents histamine-mediated vasodilation, increased vascular permeability, pruritus, and smooth muscle contraction.

Key mechanistic features:

  • Selective peripheral action: Cetirizine is a zwitterion at physiologic pH with low lipid solubility, resulting in minimal blood-brain barrier penetration and reduced CNS effects compared to first-generation antihistamines
  • No anticholinergic activity: Unlike hydroxyzine (its parent compound) and first-generation agents, cetirizine has negligible affinity for muscarinic receptors
  • Anti-inflammatory effects: Cetirizine inhibits eosinophil chemotaxis and adhesion molecule expression, reducing late-phase allergic responses
  • Inverse agonism: Rather than simply blocking histamine binding, cetirizine stabilizes the H1 receptor in its inactive conformation, reducing constitutive receptor activity

The R-enantiomer levocetirizine accounts for the majority of cetirizine's H1 receptor binding affinity.

High-yield for exams
  • Cetirizine is a second-generation H1 antihistamine with minimal anticholinergic effects and reduced CNS penetration compared to first-generation agents.
  • The drug is the carboxylic acid metabolite of hydroxyzine but has significantly less sedation and antimuscarinic activity.
  • Cetirizine is predominantly renally eliminated (70% unchanged), requiring adjustment in renal impairment.
  • Levocetirizine (R-enantiomer) has approximately double the H1 receptor affinity of racemic cetirizine.
  • Cetirizine causes more sedation than loratadine or fexofenadine but far less than first-generation antihistamines like diphenhydramine.
  • The drug has minimal CYP450-mediated metabolism, resulting in few hepatic drug-drug interactions.
  • Cetirizine must be discontinued 3-7 days before allergy skin testing to avoid false-negative results.
  • Pruritus rebound can occur after abrupt discontinuation following prolonged use, though physical dependence does not develop.

Indications

Cetirizine is indicated for the relief of symptoms associated with allergic conditions:

Primary indications:

  • Allergic rhinitis (seasonal and perennial): sneezing, rhinorrhea, pruritus, nasal congestion
  • Chronic idiopathic urticaria: pruritus and reduction in number and size of hives

Off-label uses:

  • Atopic dermatitis (adjunctive therapy for pruritus)
  • Allergic conjunctivitis
  • Physical urticarias
  • Pruritus in various dermatologic conditions

Cetirizine does not prevent or treat anaphylaxis and is not a substitute for epinephrine in severe allergic reactions.

Pharmacokinetics

Absorption

Cetirizine is rapidly absorbed after oral administration, with peak plasma concentrations reached within approximately one hour. Food decreases the rate but not the extent of absorption. Bioavailability is high, and the drug exhibits linear pharmacokinetics across the therapeutic range.

Distribution

Cetirizine is approximately 93% protein-bound in plasma. Its zwitterionic nature and low lipophilicity limit CNS penetration, though some individuals may experience sedation, particularly at higher doses. Volume of distribution is approximately 0.5 L/kg.

Metabolism

Cetirizine undergoes limited hepatic metabolism, with only about 10% metabolized via oxidative pathways. The drug is not a substrate for CYP450 enzymes, resulting in minimal drug-drug interactions via hepatic metabolism.

Elimination

Approximately 70% of cetirizine is excreted unchanged in urine, with smaller amounts eliminated in feces. The elimination half-life is approximately 8-9 hours in adults, extending to 6-12 hours in children aged 2-5 years and up to 14-18 hours in elderly patients. Renal impairment significantly prolongs half-life, requiring adjustment in patients with creatinine clearance below normal values.

Adverse effects

Cetirizine is generally well-tolerated, with most adverse effects being mild and transient.

Common adverse effects:

  • Somnolence/sedation: Most common, dose-dependent, occurs in 10-15% of patients but significantly less than first-generation antihistamines
  • Fatigue: Related to CNS effects
  • Dry mouth: Mild anticholinergic effect, less common than with first-generation agents
  • Headache: Reported in clinical trials
  • Pharyngitis: In pediatric populations

Less common adverse effects:

  • Dizziness
  • Abdominal pain
  • Diarrhea
  • Epistaxis
  • Bronchospasm (rare)

Rare but serious adverse effects:

  • Hypersensitivity reactions: Including angioedema, fixed drug eruption, anaphylaxis
  • Hepatotoxicity: Rare elevations in liver enzymes
  • Seizures: Extremely rare, primarily in predisposed individuals or overdose
  • QT prolongation: Not typically associated with cetirizine at therapeutic doses

Withdrawal phenomenon:

Abrupt discontinuation after prolonged use may unmask underlying pruritus (itch rebound), though this is not a true withdrawal syndrome. Cetirizine does not cause physical dependence.

Contraindications and warnings

Absolute contraindications:

  • Hypersensitivity to cetirizine, hydroxyzine, or any component of the formulation
  • End-stage renal disease requiring dialysis (depending on product labeling)

Warnings and precautions:

  • Renal impairment: Adjustment is necessary for creatinine clearance impairment; cetirizine is not effectively removed by dialysis
  • Hepatic impairment: Use with caution in patients with both hepatic and renal dysfunction
  • CNS depression: Enhanced with concurrent use of alcohol or other CNS depressants; caution when operating machinery or driving
  • Urinary retention: Use caution in patients with bladder neck obstruction or urinary retention, though anticholinergic effects are minimal
  • Elderly patients: Increased risk of sedation and confusion; consider reduced initial dosing

Pregnancy and lactation:

  • Pregnancy: Category B (US system, historical). No evidence of teratogenicity in animal studies; human data are reassuring but limited. Cetirizine is generally considered acceptable for use during pregnancy when benefit outweighs risk.
  • Lactation: Cetirizine is excreted in breast milk in small amounts. Drowsiness in the nursing infant is possible. Generally considered compatible with breastfeeding, but monitor infant for sedation.

Pediatric considerations:

Cetirizine is approved for use in children, but age-appropriate formulations and careful attention to weight-based considerations should guide selection according to product labeling.

Drug interactions

Cetirizine has a favorable drug interaction profile due to minimal hepatic metabolism.

Pharmacodynamic interactions:

  • CNS depressants: Alcohol, benzodiazepines, opioids, sedating antidepressants may enhance sedation
  • Anticholinergic agents: Additive anticholinergic effects, though cetirizine has minimal antimuscarinic activity

Pharmacokinetic interactions:

  • Theophylline: May decrease cetirizine clearance by approximately 16%, though this is rarely clinically significant
  • Ritonavir: May increase cetirizine exposure, but adjustment is usually not required
  • P-glycoprotein inhibitors: Theoretical interaction, as cetirizine is a P-gp substrate, but clinical relevance is unclear

Laboratory interactions:

Antihistamines should be discontinued at least 3-7 days before allergy skin testing to avoid false-negative results. Cetirizine does not interfere with most routine laboratory tests.

No clinically significant interactions:

Cetirizine does not inhibit or induce CYP450 enzymes, so interactions with drugs metabolized by these pathways are not expected.

Clinical pearls

  • Second-generation advantage: Cetirizine's zwitterionic structure reduces CNS penetration compared to first-generation antihistamines, but it still causes more sedation than fexofenadine or loratadine in some patients.
  • Levocetirizine: The R-enantiomer of cetirizine, marketed separately, has approximately twice the affinity for H1 receptors and may be effective at lower doses.
  • Renal dosing is critical: Because cetirizine is predominantly renally eliminated, failure to adjust in renal impairment can lead to drug accumulation and increased sedation.
  • Pruritus rebound: Patients stopping cetirizine after chronic use may experience temporary worsening of itching; tapering may help, though not always necessary.
  • Combination therapy: Cetirizine can be combined with nasal corticosteroids for allergic rhinitis, providing complementary mechanisms of action.
  • Non-sedating claim: While classified as "non-sedating," cetirizine causes sedation in a clinically meaningful percentage of patients (10-15%), more than other second-generation agents.
  • Urticaria management: Cetirizine may be increased beyond standard amounts in chronic urticaria refractory to standard approaches, as per specialist guidance.
  • Minimal drug interactions: The lack of CYP450 involvement makes cetirizine a safe choice in polypharmacy patients.
  • Pregnancy safety: Among antihistamines, cetirizine has reassuring human pregnancy data and is often a preferred option when treatment is necessary.
  • Once-daily dosing: The 8-9 hour half-life supports once-daily administration, improving adherence compared to older antihistamines requiring multiple daily doses.

Practice questions

1. A 28-year-old woman with seasonal allergic rhinitis is started on cetirizine for symptom relief. She returns one week later reporting that her nasal congestion, sneezing, and rhinorrhea have improved, but she feels drowsy during the day despite taking the medication only at bedtime. She works as a long-distance truck driver. Which of the following best explains her drowsiness?

Answer: C. Cetirizine, despite being a second-generation antihistamine, is a zwitterion at physiologic pH with low but not absent CNS penetration, causing sedation in 10-15% of patients. This is more than other second-generation agents like loratadine or fexofenadine but far less than first-generation antihistamines. Options A and B are incorrect because cetirizine has minimal anticholinergic activity and is classified as second-generation. Her occupation requires an alternative like fexofenadine.

2. A 65-year-old man with chronic kidney disease (creatinine clearance 25 mL/min) presents with pruritus from chronic idiopathic urticaria. His physician is considering cetirizine therapy. Which of the following pharmacokinetic properties of cetirizine is most important when prescribing this medication for this patient?

Answer: B. Cetirizine is approximately 70% renally eliminated unchanged, making adjustment essential in patients with renal impairment to avoid drug accumulation and increased sedation. The half-life is prolonged in renal dysfunction, so standard dosing would be inappropriate. Cetirizine undergoes minimal hepatic metabolism, is not a CYP450 inhibitor, has low lipophilicity limiting CNS penetration, and has an 8-9 hour half-life in normal renal function. This patient requires a reduced dose or extended dosing interval.

3. A 6-year-old boy with perennial allergic rhinitis has been taking cetirizine daily for the past 8 months with good symptom control. His mother discontinued the medication abruptly 3 days ago after reading online that long-term antihistamine use is harmful. The child now complains of intense itching all over his body, though he has no rash. His mother is concerned about withdrawal symptoms and addiction. What is the most likely explanation for his symptoms?

Answer: D. This patient is experiencing pruritus rebound, a phenomenon where itching temporarily worsens after stopping chronic antihistamine therapy as underlying histamine-mediated symptoms are unmasked. This is not true physical dependence or addiction, and cetirizine does not cause withdrawal syndromes. Symptoms typically resolve within days to weeks. Tapering may help but is not always necessary. The timing (3 days post-discontinuation), absence of rash, and previous good control make rebound the most likely explanation rather than new disease or drug reaction.

Frequently asked questions

How does cetirizine work?

Cetirizine works by selectively blocking peripheral histamine H1 receptors, preventing histamine from binding and causing allergic symptoms. As a second-generation antihistamine, it has a zwitterionic structure that limits its ability to cross the blood-brain barrier, resulting in less sedation than older antihistamines. It also has minimal anticholinergic effects and provides anti-inflammatory benefits by inhibiting eosinophil chemotaxis.

What are the side effects of cetirizine?

The most common side effect of cetirizine is mild sedation or drowsiness, occurring in 10-15% of patients. Other frequent side effects include fatigue, headache, dry mouth, and pharyngitis in children. Serious adverse effects are rare but can include hypersensitivity reactions, angioedema, and very rarely seizures in predisposed individuals. Cetirizine causes significantly less sedation and fewer anticholinergic effects than first-generation antihistamines like diphenhydramine.

Is cetirizine a controlled substance?

No, cetirizine is not a controlled substance. It is available over-the-counter in many countries, including the United States, for the treatment of allergic rhinitis and urticaria. Cetirizine does not cause physical dependence or addiction, though some patients may experience temporary rebound pruritus after stopping the medication following prolonged use. This rebound effect is not a withdrawal syndrome and does not indicate substance dependence.

Can I take cetirizine while pregnant?

Cetirizine is generally considered safe during pregnancy and was previously classified as Pregnancy Category B. Human studies have not shown an increased risk of birth defects, and animal studies have been reassuring. Cetirizine is often preferred over first-generation antihistamines during pregnancy due to its better safety profile and lower sedation risk. However, as with any medication during pregnancy, it should only be used when the benefit outweighs the risk, and you should consult your healthcare provider.

Cetirizine vs loratadine — what is the difference?

Both cetirizine and loratadine are second-generation H1 antihistamines used for allergic rhinitis and urticaria, but they differ in their side effect profiles and pharmacokinetics. Cetirizine causes more sedation (10-15% of patients) compared to loratadine, which is considered less sedating. Cetirizine is primarily renally eliminated with minimal hepatic metabolism, while loratadine undergoes extensive liver metabolism to its active form desloratadine. Both have minimal anticholinergic effects and once-daily dosing, but individual patient response varies.

Conditions it is used for

Allergic rhinitisChronic idiopathic urticariaSeasonal allergiesPerennial allergiesAllergic conjunctivitisAtopic dermatitisPruritusPhysical urticaria
levocetirizineloratadinefexofenadinedesloratadinehydroxyzinediphenhydraminechlorpheniraminebilastine

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: R06AE07

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.