Acyclovir Dosing for a 21-lb (9.5 kg) Child
For a 21-lb (9.5 kg) child, give acyclovir 200 mg/5 mL suspension at a dose of 190 mg (4.75 mL) per dose, administered three times daily for most HSV infections.
Dose Calculation Based on Indication
The appropriate dose depends on the specific herpes simplex virus (HSV) infection being treated:
For Mild Mucocutaneous HSV (Gingivostomatitis, Genital Herpes)
- Dose: 20 mg/kg per dose, three times daily 1
- For 9.5 kg child: 20 mg/kg × 9.5 kg = 190 mg per dose
- Volume: 4.75 mL of the 200 mg/5 mL suspension
- Maximum dose: 400 mg per dose
- Duration: 5-10 days
This is the most common outpatient indication and applies to children <45 kg with primary or recurrent HSV infections 1.
For Moderate-to-Severe Gingivostomatitis
If the child has severe oral HSV requiring hospitalization:
- Start with intravenous acyclovir 5-10 mg/kg per dose, three times daily 1
- Switch to oral acyclovir (20 mg/kg/dose three times daily) once lesions begin to regress
- Continue until complete healing
For CNS or Disseminated Disease (Outside Neonatal Period)
- Requires hospitalization with IV acyclovir 10 mg/kg per dose, three times daily for 21 days 1
- This is NOT appropriate for oral suspension therapy
For Neonatal HSV Disease
If this is a neonate (age <1 month):
- IV acyclovir 20 mg/kg per dose, three times daily 1
- Duration: 21 days for CNS/disseminated disease, 14 days for skin/eye/mouth disease
- Oral suspension is NOT appropriate for neonatal HSV 2
Practical Administration
Suspension preparation:
- Each 5 mL contains 200 mg acyclovir
- For 190 mg dose: measure 4.75 mL
- Can round to 5 mL (200 mg) for practical dosing without exceeding safety limits
Timing:
- Administer every 8 hours (three times daily)
- May be given with or without food 3
Important Clinical Considerations
Renal function monitoring:
- Acyclovir is primarily renally excreted 3
- Ensure adequate hydration to prevent nephrotoxicity
- Dose adjustment needed if renal impairment develops
Age-specific concerns:
- Pediatric experience with oral acyclovir in children <2 years is limited 2, 3
- The pharmacokinetics in young children show longer half-life (2.6 hours) compared to adults 3
- Bioavailability is approximately 10-20% and decreases with increasing dose 3
Common pitfalls to avoid:
- Do NOT use oral acyclovir for suspected encephalitis or disseminated disease—these require IV therapy 1, 4
- Do NOT use oral suspension for neonatal HSV—IV therapy is mandatory 2, 1
- Do NOT underdose based on adult fixed dosing (e.g., 200 mg)—pediatric dosing is weight-based at 20 mg/kg 1
When to escalate to IV therapy:
- Inability to tolerate oral medications
- Severe or worsening symptoms
- Immunocompromised status
- Any concern for CNS involvement (altered mental status, seizures, focal neurologic signs)