What are the recommended famciclovir doses for herpes zoster, genital herpes (initial and recurrent), herpes labialis, severe HSV infection in immunocompromised patients, and how should dosing be adjusted for renal impairment, pediatric patients, and pregnancy?

Famciclovir Dosing for Different Diseases

Famciclovir is dosed at 500 mg orally three times daily for 7 days for herpes zoster, 250 mg three times daily for 7-10 days for initial genital herpes, and 125 mg twice daily for 5 days for recurrent genital herpes or herpes labialis in immunocompetent adults. 1, 2, 3

Herpes Zoster (Shingles)

Standard dosing for immunocompetent adults:

• 500 mg orally three times daily for 7 days 1, 4, 5, 6
• Treatment should be initiated within 72 hours of rash onset for maximum benefit 4, 6
• This regimen has been proven to reduce the duration of postherpetic neuralgia (PHN) by approximately 3.5 months in patients ≥50 years old compared to placebo 6

Alternative dosing schedules (shown to be equally effective):

• 750 mg once daily for 7 days 7
• 500 mg twice daily for 7 days 7
• These less frequent regimens may improve compliance while maintaining comparable efficacy to the standard three-times-daily dosing 7

Genital Herpes

Initial Episode

• 250 mg orally three times daily for 7-10 days 1
• Treatment may be extended if healing is incomplete after 10 days 1
• This dosing is equivalent in efficacy to acyclovir regimens for first-episode disease 1

Recurrent Episodes

• 125 mg orally twice daily for 5 days 2, 5, 8
• Treatment must be initiated during prodrome or within 1 day of lesion onset for maximum benefit 3
• Patients should be provided with medication to enable self-initiation at first sign of recurrence 3

Suppressive Therapy

• Famciclovir is effective for chronic suppression of recurrent genital herpes 5, 8
• Consider suppressive therapy for patients with ≥6 recurrences per year 3
• Specific suppressive dosing regimens are not detailed in the provided guidelines, but the medication has demonstrated efficacy in this indication 8

Herpes Labialis (Cold Sores)

• 125 mg orally twice daily for 5 days 2, 3
• Single-dose regimens have been approved and may improve compliance 8
• Treatment must be initiated during prodrome or within 1 day of lesion onset 3
• Critical pitfall: Topical acyclovir is substantially less effective than oral therapy and should not be used 3

Immunocompromised Patients

Important limitation: Famciclovir has limited pediatric data and no specific pediatric formulation available 1

• For adolescents who can tolerate adult dosing: Use standard adult doses 1
• For severe HSV infections in immunocompromised patients: Famciclovir is not the first-line agent; IV acyclovir is preferred for severe disease 2
• Acyclovir-resistant HSV may require IV foscarnet or cidofovir 1

Renal Impairment

• Dose adjustment is mandatory based on creatinine clearance 9
• Famciclovir is predominantly eliminated via the kidneys with a half-life of approximately 2.5 hours 5
• For patients with severe renal impairment who cannot tolerate adjusted oral dosing, IV acyclovir with appropriate dose reduction may be necessary 9

Pediatric Patients

• No pediatric preparation is available, and data on dosing in children are limited 1
• Famciclovir could be used by older children and adolescents able to receive adult dosing 1
• For children <12 years or those unable to swallow tablets, acyclovir remains the preferred alternative at 20 mg/kg orally four times daily (maximum 800 mg/dose) 9

Pregnancy

• No specific recommendations for famciclovir use in pregnancy are provided in the guidelines 1
• The guidelines note that acyclovir has no pattern of adverse pregnancy outcomes reported 1
• For pregnant patients requiring antiviral therapy, consultation with experts is recommended, as acyclovir has more established safety data in pregnancy 1

Key Clinical Considerations

Advantages of famciclovir over acyclovir:

• Better oral bioavailability (77% vs. lower for acyclovir) 5
• Less frequent dosing (three times daily vs. five times daily for acyclovir in herpes zoster) 4, 5, 6
• Longer intracellular half-life of penciclovir triphosphate (7 hours vs. 1 hour for acyclovir) 7
• Comparable efficacy with improved convenience and potentially better compliance 4, 8

Common adverse effects:

• Headache and gastrointestinal upset are most common 5
• Constipation, nausea, and vomiting have been reported 4
• Well-tolerated with a safety profile comparable to acyclovir 4, 8

Critical timing: For all indications, early initiation of therapy (within 72 hours of symptom onset for zoster, within 1 day for recurrent HSV) is essential for optimal outcomes 3, 4, 6