Antiviral — HSV Encephalitis / Neonatal Herpes / VZV in Immunocompromised Pregnancy: No formal category is stated. A post-marketing aciclovir pregnancy registry has not shown an increase in birth defects compared with the general population, and no consistent pattern suggesting a common cause; systemic aciclovir was not embryotoxic or teratogenic in standard animal tests. 'Caution should therefore be exercised by balancing the potential benefits of treatment against any possible hazard.' Breast-feeding: after oral 200 mg five times daily, aciclovir has been detected in human breast milk at 0.6 to 4.1 times the corresponding plasma levels, potentially exposing nursing infants to up to 0.3 mg/kg/day — caution is advised in nursing women (§4.6).

Aciclovir (Paediatric)

Brand names: Zovirax

This page covers aciclovir use in children — an antiviral active against herpes simplex and varicella-zoster, used for conditions such as neonatal herpes, herpes encephalitis, chickenpox and zoster.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 5 mg/kg bodyweight every 8 hours for Herpes simplex (except herpes encephalitis) or Varicella zoster infections; 10 mg/kg bodyweight every 8 hours for immunocompromised patients with Varicella zoster infections or patients with herpes encephalitis (both provided renal function is not impaired)

Route: Slow intravenous infusion over 1 hour, with adequate hydration established; may be given by a controlled-rate infusion pump. Rapid or bolus injection must be avoided and the solution (pH approximately 11) must not be given by any other route — 'Aciclovir should not be administered by mouth' (§4.4)

Frequency: Every 8 hours

SOURCE: eMC UK SPC, productName 'Aciclovir 25 mg/ml Concentrate for solution for infusion', §4.2. FORMULATION CAVEAT: this is the INTRAVENOUS concentrate SPC — it contains no oral aciclovir regimens, so oral dosing for this page must be sourced separately. VERBATIM: 'Patients with Herpes simplex (except herpes encephalitis) or Varicella zoster infections should be given ... 5 mg/kg bodyweight every 8 hours provided renal function is not impaired' and 'Immunocompromised patients with Varicella zoster infections or patients with herpes encephalitis should be given ... 10 mg/kg bodyweight every 8 hours provided renal function is not impaired.' DURATION: a course usually lasts 5 days but may be adjusted to the patient's condition and response; herpes encephalitis usually 10 days; neonatal herpes infections usually 14 days for mucocutaneous (skin-eye-mouth) infections and 21 days for disseminated or central nervous system disease; duration of prophylaxis is determined by the duration of the period at risk. OBESITY: dosing on actual body weight may give higher plasma concentrations — consider dose reduction in obese patients, especially with renal impairment or in the elderly. ELDERLY: consider the possibility of renal impairment and adjust dose accordingly; maintain adequate hydration. Source §4.4/§4.5 text was truncated at the source-fetch limit.

Paediatric dose

Route: Slow intravenous infusion over 1 hour (never by bolus injection or by mouth)

Frequency: Every 8 hours

dosePerKg is left null because paediatric dosing in this SPC is calculated on BODY SURFACE AREA for infants and children aged 3 months to 12 years, and a per-kg figure is stated ONLY for neonates and infants up to 3 months — carrying a single mg/kg number would risk it being applied to the wrong age band. VERBATIM §4.2 'Paediatric Population': infants and children 3 months of age or older with Herpes simplex (except herpes encephalitis) or Varicella zoster — '250 mg per square metre of body surface area every 8 hours if renal function is not impaired'; immunocompromised children with Varicella zoster infections or children with herpes encephalitis — '500 mg per square metre body surface area every 8 hours if renal function is not impaired'. NEONATES AND INFANTS UP TO 3 MONTHS are dosed on body weight — 'The recommended regimen for infants treated for known or suspected neonatal herpes is acyclovir 20 mg/kg body weight IV every 8 hours for 21 days for disseminated and CNS disease, or for 14 days for disease limited to the skin and mucous membranes.' Infants and children with impaired renal function require an appropriately modified dose according to the degree of impairment (see renalAdjustment). No paediatric maximum dose is stated in this SPC. Verify all under-18 dosing against a children's formulary.

Dose adjustments

Renal

Caution is advised in impaired renal function and adequate hydration must be maintained; adjustment is based on creatinine clearance (ml/min for adults and adolescents, ml/min/1.73 m2 for infants and children under 13 years). ADULTS AND ADOLESCENTS: creatinine clearance 25–50 ml/min — give the dose recommended above (5 or 10 mg/kg) every 12 hours; 10–25 ml/min — every 24 hours; 0 (anuric) to 10 ml/min — on CAPD, halve the recommended dose and give every 24 hours; on haemodialysis, halve the recommended dose and give every 24 hours and after dialysis. INFANTS AND CHILDREN: creatinine clearance 25–50 ml/min/1.73 m2 — give the dose recommended above (250 or 500 mg/m2 or 20 mg/kg) every 12 hours; 10–25 ml/min/1.73 m2 — every 24 hours; 0–10 ml/min/1.73 m2 — on CAPD, halve the dose and give every 24 hours; on haemodialysis, halve the dose and give every 24 hours and after dialysis. Elderly patients and patients with renal impairment are at increased risk of neurological side effects and should be closely monitored (§4.4).

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to aciclovir and valaciclovir or to any of the excipients

Side effects

Phlebitis (common); severe local inflammatory reactions, sometimes leading to breakdown of the skin, have occurred when the infusion has inadvertently entered extracellular tissues (very rare)
Nausea and vomiting (common); diarrhoea and abdominal pain (very rare)
Increases in blood urea and creatinine (common), believed to relate to peak plasma levels and hydration state — avoid IV bolus and infuse over one hour; renal impairment, acute renal failure and renal pain (very rare)
Pruritus, urticaria and rashes including photosensitivity (common); angioedema (very rare); reversible increases in liver-related enzymes (common)
Neurological reactions — headache, dizziness, agitation, confusion, tremor, ataxia, dysarthria, hallucinations, psychotic symptoms, convulsions, somnolence, encephalopathy and coma (very rare); generally reversible and usually reported in patients with renal impairment or other predisposing factors. Anaphylaxis (very rare)

Interactions

Drugs that compete with active renal tubular secretion — aciclovir is eliminated primarily unchanged in the urine via active renal tubular secretion, and any drug administered concurrently that competes with this mechanism may increase aciclovir plasma concentrations (§4.5, opening statement)
Other nephrotoxic drugs — 'The risk of renal impairment is increased by use with other nephrotoxic drugs. Care is required if administering i.v aciclovir with other nephrotoxic drugs' (§4.4)
NOTE: SPC §4.5 was truncated at the source-fetch limit in this bundle — the clinician must verify the full interaction list before publishing

Clinical monograph

How it works

Aciclovir is phosphorylated by viral thymidine kinase and cellular enzymes to a triphosphate that inhibits viral DNA polymerase and terminates viral DNA chain elongation.

Prescribing in practice

In children, doses must be calculated against weight or body-surface area using a children's formulary, and intravenous aciclovir requires adequate hydration to prevent crystal nephropathy and acute kidney injury.
Dose adjustment is needed in renal impairment, and infusion should be given slowly rather than as a bolus.
Neurotoxicity such as confusion or tremor can occur, particularly with high plasma levels or impaired renal function.

Monitoring

Monitor renal function and hydration during intravenous therapy, and observe for neurological side effects.

Counselling the patient

Ensure the child drinks well or receives fluids during treatment.
Starting treatment early gives the best chance of benefit.
Report reduced urine output, drowsiness or unusual behaviour.

Evidence & guidelines

Aciclovir is the established first-line antiviral for serious paediatric herpes infections in UK guidance.

Reference: NICE NG41 (Meningitis); PHE Neonatal Herpes Guidelines; BPSU/BPNG Neonatal Herpes Simplex Guidelines; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways