What is the recommended dosing interval for hydroxyzine (antihistamine)?

Hydroxyzine Dosing Intervals

According to the FDA label, hydroxyzine should be administered three to four times daily (t.i.d. or q.i.d.) for most indications, with specific dosing of 25 mg t.i.d. or q.i.d. for adults when used for pruritus and allergic conditions. 1

Dosing Schedule Based on Indication

For Pruritus and Allergic Conditions:

• Adults: 25 mg three to four times daily (t.i.d. or q.i.d.) 1
• Children under 6 years: 50 mg daily in divided doses 1
• Children over 6 years: 50-100 mg daily in divided doses 1

For Anxiety and Tension:

• Adults: 50-100 mg four times daily (q.i.d.) 1
• Children under 6 years: 50 mg daily in divided doses 1
• Children over 6 years: 50-100 mg daily in divided doses 1

For Sedation (Premedication):

• Adults: 50-100 mg as a single dose 1
• Children: 0.6 mg/kg body weight 1

Pharmacokinetic Considerations

Hydroxyzine has a long half-life, particularly in elderly patients where studies have shown a serum elimination half-life of approximately 29.3 hours 2. This extended half-life results in prolonged effects:

• Wheal and flare suppression (antihistamine effect) can last from 1 to 144 hours after a single dose in elderly patients 2
• Maximum wheal suppression occurs between 4-10 hours post-dose 2
• Maximum flare suppression occurs between 2-72 hours post-dose 2

Despite this long half-life, the standard dosing remains divided throughout the day to maintain consistent blood levels.

Special Populations

Elderly Patients:

• Consider reduced dosing frequency due to prolonged half-life (29.3 ± 10.1 hours) 2
• Elderly patients are more sensitive to psychomotor impairment and anticholinergic effects 3

Patients with Renal Impairment:

• Dose should be reduced in moderate to severe renal impairment 4
• Avoid in severe renal impairment (creatinine clearance <10 mL/min) 4

Patients with Hepatic Impairment:

• Starting dose should be half the standard dose 3
• Monitor closely for side effects 3

Side Effect Considerations

The sedative effects of hydroxyzine are significant and may impact dosing decisions:

• Hydroxyzine significantly prolongs both simple and choice reaction times compared to non-sedating antihistamines 5
• Sleepiness is a common side effect (28% vs 14% with placebo) that typically appears during the first week of treatment 6
• Daytime sleepiness is more frequent with higher doses (50 mg) 7

Clinical Pearls

• Avoid morning/daytime dosing when alertness is required due to significant sedation potential 3
• For nighttime pruritus, a single bedtime dose may be preferable to take advantage of the sedative effects 3
• The long half-life means effects can persist well beyond the dosing interval, especially in elderly patients 2
• Tolerance to the sedative effects may develop over time, but not to the antihistamine effects 5

Remember that hydroxyzine's dosing should be adjusted according to the patient's response to therapy, as noted in the FDA label 1, with consideration of the significant sedative effects that may limit daytime use.