Pediatric Dose of Acyclovir for Cold Sores (Herpes Labialis)
For pediatric cold sores (herpes labialis/gingivostomatitis), use oral acyclovir 20 mg/kg per dose (maximum 400 mg/dose) three times daily for 5-10 days, starting treatment as early as possible for maximum benefit. 1
Dosing by Severity
Mild Symptomatic Gingivostomatitis (Cold Sores)
- Acyclovir 20 mg/kg per dose orally (maximum 400 mg/dose) three times daily for 5-10 days 1
- This is the standard outpatient regimen for typical cold sores in children
- Treatment should be initiated as soon as lesions appear for optimal efficacy 1
Moderate to Severe Gingivostomatitis
- Start with IV acyclovir 5-10 mg/kg per dose three times daily 1
- Once lesions begin to regress, transition to oral acyclovir at the same weight-based dosing 1
- Continue therapy until lesions completely heal 1
Age-Specific Considerations
Children Under 45 kg
- Use the weight-based dosing: 20 mg/kg per dose (max 400 mg/dose) orally three times daily 1
- This applies to most pediatric patients with cold sores
Adolescents and Adults
- Can use adult dosing: 400 mg orally twice daily for 5-14 days 1
- This simplified regimen is appropriate for older adolescents who can reliably take adult doses
Important Clinical Caveats
Timing is critical: The FDA label and clinical trials demonstrate that acyclovir is most effective when started within 24 hours of rash onset 2, 3. Delayed treatment significantly reduces efficacy.
Bioavailability in young children: Oral acyclovir has relatively low bioavailability (approximately 12%) in pediatric patients 4. Despite this, the recommended doses achieve therapeutic concentrations for HSV infections 5.
Renal function monitoring: Acyclovir clearance is dependent on renal function 2. For children with impaired renal function (eGFR <250 ml/min/1.73 m²), dosage adjustment is necessary 5. Conversely, children with augmented renal clearance (eGFR >250 ml/min/1.73 m²) may require higher or more frequent dosing 5.
Food administration: Acyclovir may be administered with or without food, as food does not affect absorption 2.
Alternative Agents for Older Children
Valacyclovir (if able to swallow tablets and receive adult dosing):
- 1000 mg orally three times daily for 7 days 6
- Provides enhanced acyclovir bioavailability compared to oral acyclovir 7
- No pediatric liquid formulation exists; limited data for children under 12 years 1
Famciclovir (for adolescents):
Safety Profile
Acyclovir is well-tolerated in pediatric patients 2, 3. The most common adverse events in infants and children include thrombocytopenia (25% of patients) and hypotension (9%), though many adverse events may be related to the underlying infection rather than the medication 8. Elevated creatinine occurs in only 2% of infants, and renal failure requiring dialysis is extremely rare 8. Treatment does not interfere with normal antibody development or immune memory formation 9, 2.