What is the maximum recommended daily dose of acyclovir (antiviral medication) for a 75kg adult with impaired renal function?

Maximum Daily Dose of Acyclovir in a 75kg Adult

For a 75kg adult with normal renal function, the maximum daily dose of acyclovir is 4,000 mg per day (800 mg five times daily), used for treating herpes zoster or chickenpox; however, in patients with impaired renal function, dose reduction is mandatory based on creatinine clearance to prevent neurotoxicity and nephrotoxicity. 1

Dosing by Indication in Adults with Normal Renal Function

The maximum daily dose varies by clinical indication 1:

• Herpes Zoster (Shingles): 800 mg orally every 4 hours, 5 times daily = 4,000 mg/day for 7-10 days 1, 2
• Chickenpox: 800 mg orally 4 times daily = 3,200 mg/day for 5 days 1
• Initial Genital Herpes: 200 mg every 4 hours, 5 times daily = 1,000 mg/day for 10 days 3, 1
• Recurrent Genital Herpes: Maximum of 800 mg twice daily = 1,600 mg/day for 5 days 3
• Suppressive Therapy: 400 mg twice daily = 800 mg/day 3, 1

Critical Dose Adjustments for Renal Impairment

Dose reduction is essential in renal impairment to avoid serious adverse effects. 1 For a 75kg adult, adjustments are based on creatinine clearance:

For 800 mg Every 4 Hours Regimen (Herpes Zoster/Chickenpox):

• CrCl >25 mL/min: 800 mg every 4 hours, 5 times daily (4,000 mg/day) 1
• CrCl 10-25 mL/min: 800 mg every 8 hours (2,400 mg/day) 1
• CrCl 0-10 mL/min: 800 mg every 12 hours (1,600 mg/day) 1

For 400 mg Every 12 Hours Regimen (Suppressive Therapy):

• CrCl >10 mL/min: 400 mg every 12 hours (800 mg/day) 1
• CrCl 0-10 mL/min: 200 mg every 12 hours (400 mg/day) 1

For 200 mg Every 4 Hours Regimen (Initial Genital Herpes):

• CrCl >10 mL/min: 200 mg every 4 hours, 5 times daily (1,000 mg/day) 1
• CrCl 0-10 mL/min: 200 mg every 12 hours (400 mg/day) 1

Special Considerations for Dialysis Patients

Hemodialysis: The plasma half-life extends to approximately 5 hours during dialysis, with a 60% decrease in plasma concentrations after a 6-hour session 1. An additional dose must be administered after each dialysis session 1.

Peritoneal Dialysis: No supplemental dose is required after adjusting the dosing interval 1.

Intravenous Dosing for Severe Disease

For severe herpes infections requiring hospitalization (disseminated infection, encephalitis, pneumonitis, or hepatitis), intravenous acyclovir is dosed at 5-10 mg/kg every 8 hours 3. For a 75kg adult, this translates to:

• 375-750 mg every 8 hours = 1,125-2,250 mg/day 3
• For herpes zoster in immunocompromised patients: 500 mg/m² or 10 mg/kg every 8 hours 2

Age-Related Considerations

Geriatric patients (≥65 years) experience higher acyclovir plasma concentrations due to age-related decline in renal function 1, 4. Studies show 15-20% higher peak concentrations and 30-50% higher drug exposure compared to younger adults 4. Dose reduction may be required even without documented renal impairment 1.

Critical Safety Warnings

Adequate hydration and urine flow must be maintained at high doses (≥3,200 mg/day) to prevent crystalluria and nephrotoxicity 2. The prevalence of acyclovir-associated nephrotoxicity ranges from 13-21% 5.

Monitor mental status closely as neurotoxicity can occur, particularly in patients with renal impairment receiving inadequately adjusted doses 2.

Common pitfall: Using ideal body weight in obese patients may result in subtherapeutic concentrations, while using actual body weight may cause toxicity 5. For obese patients with normal renal function, consider using adjusted body weight for dosing calculations 5.