What are the differences between hydroxyzine pamoate and hydroxyzine hydrochloride?

Hydroxyzine Pamoate vs. Hydroxyzine Hydrochloride: Key Differences

Hydroxyzine pamoate and hydroxyzine hydrochloride are pharmacologically equivalent formulations that differ only in their salt form and route of administration—pamoate is an oral capsule formulation while hydrochloride is available in both oral (tablet/syrup) and injectable forms. 1

Chemical and Pharmaceutical Differences

Salt Formulation

• Hydroxyzine pamoate is the pamoate salt (1'-methylene bis[2-hydroxy-3-naphthalene carboxylic acid]) with a molecular weight of 763.27, formulated exclusively as oral capsules in 25 mg, 50 mg, and 100 mg strengths 1
• Hydroxyzine hydrochloride is the hydrochloride salt, available in multiple oral formulations (tablets, syrup) and as an injectable solution 1, 2

Bioequivalence

• Both formulations deliver equivalent doses of the active hydroxyzine molecule—for example, 25 mg of hydroxyzine pamoate provides the same therapeutic effect as 25 mg of hydroxyzine hydrochloride 1
• The FDA label explicitly states that hydroxyzine pamoate capsules are "administered in doses equivalent to 25 mg, 50 mg or 100 mg of hydroxyzine hydrochloride" 1

Clinical Pharmacology (Identical for Both Formulations)

Absorption and Onset

• Both formulations are rapidly absorbed from the gastrointestinal tract, with clinical effects typically noted within 15-30 minutes after oral administration 1
• In elderly patients (mean age 69.5 years), the elimination half-life is approximately 29.3 hours, with a large volume of distribution (22.5 L/kg) 2

Duration of Action

• A single dose produces prolonged antihistaminic effects, with significant wheal and flare suppression lasting 1-144 hours after administration 2
• Maximum wheal suppression occurs 4-10 hours post-dose, while maximum flare suppression extends from 2-72 hours 2

Active Metabolite

• Both formulations generate cetirizine as an active metabolite in vivo, with a half-life of 24.8 hours (not significantly different from the parent compound) 2

Practical Clinical Implications

Formulation Selection

• Choose hydroxyzine pamoate when oral capsule administration is preferred and the patient can swallow capsules 1
• Choose hydroxyzine hydrochloride when:
  • Liquid formulation (syrup) is needed for patients who cannot swallow capsules 2
  • Parenteral (injectable) administration is required in acute settings 1
  • Tablet formulation is preferred over capsules 1

Dosing Equivalence

• Prescribe identical milligram doses regardless of salt form—if a patient requires 50 mg at bedtime for pruritus, either 50 mg hydroxyzine pamoate capsule or 50 mg hydroxyzine hydrochloride tablet/syrup will provide equivalent therapeutic effect 3, 1

Common Pitfall to Avoid

• Do not adjust doses when switching between pamoate and hydrochloride formulations—the labeled strengths already account for salt differences and provide bioequivalent amounts of active hydroxyzine 1
• Both formulations carry identical safety profiles, including 80% sedation rates, significant performance impairment, anticholinergic effects, and driving impairment (1.5-fold increased risk of fatal accidents) 3, 4

Renal and Hepatic Dosing (Applies to Both Formulations)

• Moderate renal impairment (CrCl 10-20 mL/min): Reduce dose by 50% 3
• Severe renal impairment (CrCl <10 mL/min): Avoid both formulations 3
• Severe hepatic disease: Contraindicated for both formulations due to excessive sedation risk 3

Adverse Event Profile (Identical)

Both formulations produce the same adverse effects because they deliver equivalent amounts of hydroxyzine:

• Sedation in approximately 80% of patients, with objective performance impairment persisting even when patients deny drowsiness 3
• Anticholinergic effects including dry mouth, urinary retention, constipation, blurred vision, and increased intraocular pressure 3
• In elderly patients, markedly increased risk of falls, fractures, cognitive impairment, and delirium 3
• No tolerance develops to sedative effects with chronic use 3

Cross-Reactivity in Drug Eruptions

• A case report documented positive patch tests to both hydroxyzine pamoate and hydroxyzine hydrochloride in a patient who developed drug eruption, confirming that hypersensitivity reactions are directed against the hydroxyzine molecule itself rather than the specific salt form 5
• Patients with documented allergy to one formulation should avoid both 5