What is the recommended dosage of acyclovir for an adult patient over 50 years old presenting with herpes zoster lesions?

Acyclovir Dosing for Herpes Zoster in Adults Over 50

For an adult patient over 50 years old with herpes zoster, the recommended dosage is acyclovir 800 mg orally five times daily for 7-10 days, initiated within 72 hours of rash onset (ideally within 48 hours for maximum benefit). 1, 2

Standard Oral Dosing Regimen

• Acyclovir 800 mg orally 5 times daily for 7-10 days is the established treatment for herpes zoster in immunocompetent adults 1, 2, 3
• The higher dose of 800 mg (compared to the 200-400 mg used for herpes simplex) is necessary because varicella zoster virus is less sensitive to acyclovir than herpes simplex virus 3
• Treatment must be started within 72 hours of rash onset, with greatest efficacy when initiated within 48 hours 2, 4

Critical Timing Considerations

• Adults over 50 years of age show the greatest benefit from early antiviral treatment 2
• Starting therapy after 48 hours significantly reduces effectiveness, particularly for accelerating rash healing 4
• Early treatment shortens time to lesion scabbing, healing, complete cessation of pain, and reduces duration of viral shedding 2

Expected Clinical Benefits

• Acyclovir 800 mg five times daily significantly reduces time to last new lesion formation, loss of vesicles, and full crusting 4
• Treatment decreases acute pain during the active infection phase 2, 4
• The regimen reduces the prevalence of localized zoster-associated neurologic symptoms (paresthesia, dysesthesia, hyperesthesia) 2

Intravenous Therapy for Severe Disease

• For severe disease, complications requiring hospitalization, or disseminated infection, use acyclovir 5-10 mg/kg (or 500 mg/m²) IV every 8 hours for 5-7 days 1, 3, 5
• IV therapy is indicated for patients with encephalitis, pneumonitis, hepatitis, or other visceral complications 1
• At these higher IV doses, adequate hydration and urine flow must be maintained, mental status monitored, and dosage adjusted downward for impaired renal function 3

Special Populations Requiring Modified Dosing

• Immunocompromised patients require more aggressive therapy with higher doses or IV administration 1
• Immunocompromised patients may need suppressive therapy with acyclovir 400 mg orally 3-5 times daily 1
• Geriatric patients may require dosage reduction due to age-related changes in renal function, as acyclovir plasma concentrations are higher in this population 2
• Dosage adjustment is mandatory for patients with reduced renal function, as acyclovir half-life and total body clearance are dependent on renal function 2

Important Caveats and Pitfalls

• Never use topical acyclovir for herpes zoster—it is substantially less effective than oral therapy 1
• Acyclovir does not eradicate latent virus and does not prevent future recurrences 1
• The evidence for preventing postherpetic neuralgia with acyclovir is limited; while some studies show promising trends with higher doses, no clear prevention has been definitively proven 3, 4, 5
• Adequate hydration is essential at the 800 mg five times daily dosing to prevent renal complications 3

Alternative Agents with Superior Convenience

• Valacyclovir 1000 mg three times daily for 7 days offers equivalent or superior efficacy with more convenient dosing 6, 7
• Valacyclovir significantly accelerates resolution of zoster-associated pain compared to acyclovir (median 38 days versus 51 days) and reduces duration of postherpetic neuralgia 6
• The three-times-daily valacyclovir regimen may improve adherence compared to the five-times-daily acyclovir schedule 6, 7