What is the appropriate acyclovir dosing regimen for healthy patients aged 12 years and older, children, immunocompromised patients, pregnant women, and patients with renal impairment?

Acyclovir Dosing

For healthy patients aged ≥12 years with varicella or herpes zoster, oral acyclovir 800 mg five times daily for 5-7 days is the standard regimen, initiated within 24 hours of rash onset for maximum benefit. 1

Dosing by Patient Population and Indication

Healthy Patients ≥12 Years and Adults

Varicella (Chickenpox):

• 800 mg orally 4 times daily for 5 days 1
• Must initiate within 24 hours of rash onset 2
• Note: Acyclovir is NOT routinely recommended for healthy children with varicella due to limited clinical benefit, but should be considered for those ≥12 years who are at increased risk for moderate-to-severe disease 2

Herpes Zoster (Shingles):

• 800 mg orally every 4 hours (5 times daily) for 7-10 days 1
• Initiate within 72 hours of rash onset 2

Genital Herpes:

• Initial episode: 200 mg every 4 hours (5 times daily) for 10 days 1
• Recurrent episodes: 200 mg every 4 hours (5 times daily) for 5 days 1
• Chronic suppression: 400 mg twice daily (can continue up to 12 months) 1

HSV Encephalitis:

• 10 mg/kg IV every 8 hours for 14-21 days 3, 4
• Higher doses (20 mg/kg IV every 8 hours for 21 days) have shown improved outcomes in neonates 3
• Consider repeat CSF PCR at end of therapy; continue treatment if positive 4

Children (Healthy)

Varicella:

• Ages 2-12 years: 20 mg/kg orally 4 times daily (maximum 800 mg/dose) for 5 days 1
• Children >40 kg: Use adult dose of 800 mg 4 times daily 1
• Initiate within 24 hours of rash onset 2
• Important caveat: AAP does not recommend routine acyclovir for healthy children with varicella, as clinical benefit is modest (reduces fever duration and lesion formation but does not prevent complications or reduce transmission) 2

HSV Gingivostomatitis:

• Moderate-to-severe: 5-10 mg/kg IV every 8 hours, then switch to oral when improving 5
• Mild: 20 mg/kg orally (max 400 mg/dose) 3 times daily for 5-10 days 5

Genital Herpes (children <45 kg):

• 20 mg/kg orally (max 400 mg/dose) 3 times daily for 5-14 days 5

HSV Encephalitis/CNS Disease:

• 10 mg/kg IV every 8 hours for 21 days 5
• For neonatal CNS disease, repeat CSF HSV DNA PCR at days 19-21; do not stop until negative 5
• Children 3 months-12 years should receive minimum 21 days due to higher relapse rates (up to 29%) 4

Immunocompromised Patients

Varicella:

• 10 mg/kg IV every 8 hours (intravenous route mandatory) 2
• Initiate within 24 hours of rash onset to reduce morbidity and mortality 2
• Oral acyclovir is NOT adequate for immunocompromised patients 6, 7

Herpes Zoster:

• 10 mg/kg IV every 8 hours 7
• Decreases new lesion formation, acute pain, viral dissemination, and visceral complications 7

HSV Suppression (HIV-infected):

• Frequent/severe recurrences: Daily oral acyclovir or famciclovir 8
• Valacyclovir is an alternative 8
• For acyclovir-resistant HSV: Foscarnet 40 mg/kg IV every 8 hours or 60 mg/kg IV twice daily 8, 5

Pregnant Women

Varicella with Complications (e.g., pneumonia):

• Intravenous acyclovir should be considered 2
• Acyclovir is FDA Category B; registry data from 596 first-trimester exposures showed no increased birth defect rates 2

Genital HSV:

• Routine prophylaxis in late pregnancy is controversial and NOT routinely recommended 8
• For frequent severe recurrences: Acyclovir prophylaxis may be indicated 8
• No adverse pregnancy outcomes reported with acyclovir exposure 8

Renal Impairment

Dose adjustments are MANDATORY to prevent neurotoxicity (confusion, hallucinations, seizures) and nephrotoxicity (crystalluria, obstructive nephropathy) 1, 9:

For 200 mg every 4 hours regimen:

• CrCl >10 mL/min: 200 mg every 4 hours (5x daily)
• CrCl 0-10 mL/min: 200 mg every 12 hours 1

For 400 mg every 12 hours regimen:

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

For 800 mg every 4 hours regimen:

• CrCl >25 mL/min: 800 mg every 4 hours (5x daily)
• CrCl 10-25 mL/min: 800 mg every 8 hours
• CrCl 0-10 mL/min: 800 mg every 12 hours 1

Hemodialysis patients:

• Administer additional dose after each dialysis session 1
• Recommended modified regimen: Loading dose 400 mg, then 200 mg twice daily, with 400 mg after dialysis 9
• Standard 800 mg twice daily causes neurotoxicity in dialysis patients 9

Peritoneal dialysis:

• No supplemental dose needed after interval adjustment 1

Critical Clinical Pearls

Timing is everything: Acyclovir must be initiated within 24 hours of varicella rash onset or 72 hours of zoster rash onset for meaningful clinical benefit 2.

Hydration is mandatory: Maintain adequate hydration and urine flow to prevent crystalluria and obstructive nephropathy, which can occur in up to 20% of patients after 4 days of IV therapy 4, 1.

Monitor renal function: Check baseline and serial creatinine, especially with IV therapy or in elderly patients 1, 9.

Neonates require special dosing: Preterm infants have immature renal function requiring lower doses based on postmenstrual age 10.

Encephalitis requires prolonged therapy: 14-21 days minimum, with consideration for repeat CSF PCR before stopping 3, 4. Children 3 months-12 years should receive 21 days due to higher relapse risk 4.

Oral acyclovir has poor bioavailability: Only ~20% absorbed orally 11, 12. IV route is mandatory for severe disease, immunocompromised patients, and children <2 years 6.