What oral aciclovir dose should be initiated for an adult with herpes zoster?

Oral Aciclovir Dosing for Herpes Zoster

For immunocompetent adults with uncomplicated herpes zoster, initiate oral aciclovir 800 mg five times daily for 7–10 days, starting within 72 hours of rash onset. 1, 2

Standard Dosing Regimen

• Dose: 800 mg orally every 4 hours (five times daily) 2
• Duration: 7–10 days 1, 2
• Timing window: Must be initiated within 72 hours of rash onset; efficacy declines markedly after this window 1
• Treatment endpoint: Continue until all lesions have completely scabbed, not merely for a fixed 7-day period 1

Clinical Criteria for Oral Therapy

Oral aciclovir is appropriate only when all of the following criteria are met:

• Patient is immunocompetent 1
• Involvement of 1–2 contiguous dermatomes only 1
• No visceral organ involvement (hepatitis, pneumonia) 1
• No central nervous system complications (encephalitis, meningitis, myelitis) 1
• No ophthalmic involvement or acute retinal necrosis 1

Absolute Indications to Switch to Intravenous Aciclovir

Immediately escalate to IV aciclovir 10 mg/kg every 8 hours in the following scenarios:

• Disseminated disease: ≥3 dermatomes involved, hemorrhagic or necrotic lesions, or visceral organ involvement 1
• Severe immunosuppression: Active chemotherapy, low CD4 counts, solid-organ or bone-marrow transplant recipients, chronic high-dose corticosteroids (>40 mg prednisone equivalent), or biologic immunosuppressants 1
• Complicated disease: CNS involvement (encephalitis, meningitis, myelitis, Guillain-Barré syndrome), intra-ocular VZV infection, acute retinal necrosis, or severe facial zoster with cranial nerve involvement 1
• Treatment failure: Lesions do not improve or continue to appear after 7–10 days of adequate oral therapy 1

Renal Dose Adjustment

Critical: Adjust dosing in renal impairment to prevent crystalluria and acute renal failure 1, 2:

Creatinine Clearance	Adjusted Dose	Dosing Interval
>25 mL/min/1.73 m²	800 mg	Every 4 hours (5× daily)
10–25 mL/min/1.73 m²	800 mg	Every 8 hours
0–10 mL/min/1.73 m²	800 mg	Every 12 hours

• Hemodialysis patients: Administer an additional 800 mg dose after each dialysis session 2

Evidence Supporting This Regimen

• The FDA-approved dosing for herpes zoster is 800 mg five times daily for 7–10 days 2
• A randomized controlled trial in 205 elderly immunocompetent patients demonstrated that aciclovir 800 mg five times daily significantly reduced time to arrest of new lesion formation (p=0.005), loss of vesicles (p<0.001), and full crusting (p=0.02) when started within 48 hours of rash onset 3
• The same trial showed significant reduction in pain during treatment (p=0.008), with 40% of patients with severe pain on entry having no or only mild pain at treatment end, versus 0% in placebo 3
• Comparative trials show that famciclovir (250 mg three times daily) and valaciclovir (1,000 mg three times daily) are equally effective alternatives with simpler dosing schedules 4, 5, 6, but aciclovir remains the standard when cost is a consideration

Common Pitfalls to Avoid

• Do not use genital herpes dosing: The 400 mg three times daily regimen for genital HSV is inadequate for VZV infection 1
• Do not use topical aciclovir: Topical formulations are substantially less effective and should not be used for herpes zoster 7, 1
• Do not stop prematurely: Continue therapy until all lesions are fully scabbed, even if this extends beyond 7 days 1
• Do not delay IV escalation in immunocompromised patients: Even if disease appears localized, IV therapy should be initiated immediately in severely immunocompromised hosts due to high risk of dissemination 1
• Do not confuse treatment windows: The 72-hour window for herpes zoster differs from the 24-hour window for varicella (chickenpox) 1, 8

Monitoring and Follow-Up

• Expect clinical improvement within 48–72 hours of initiating therapy 1
• If lesions persist or new lesions appear after 7–10 days: Suspect aciclovir resistance, obtain viral culture with susceptibility testing, and switch to foscarnet 40 mg/kg IV every 8 hours 1
• Resistance epidemiology: Rare in immunocompetent patients but occurs in up to 7% of immunocompromised individuals 1

Special Populations

• Pregnant women with uncomplicated shingles: Oral aciclovir may be used (FDA Pregnancy Category B), though data are limited 1
• Pregnant women with serious complications (e.g., pneumonia): IV aciclovir is recommended despite Category B status 1