Antiviral Pregnancy: Use only when potential benefits outweigh possible unknown risks; pregnancy registry has not shown an increase in birth defects.

Aciclovir

Brand names: Zovirax

Used in: Meningitis & Encephalitis Acute Red Eye

Aciclovir is an antiviral used to treat herpes simplex and varicella-zoster infections, available by oral, intravenous and topical routes.

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: 200 mg

Route: Oral

Frequency: Five times daily at approximately four-hourly intervals (omitting the night-time dose)

Treatment of herpes simplex infections: 200 mg five times daily for 5 days (may be extended in severe initial infections); dose can be doubled to 400 mg in severely immunocompromised patients or impaired gut absorption. Suppression of HSV (immunocompetent): 200 mg four times daily, or 400 mg twice daily. Prophylaxis of HSV (immunocompromised): 200 mg four times daily. Treatment of varicella and herpes zoster: 800 mg five times daily for 7 days. Begin dosing as early as possible after infection onset.

Paediatric dose

Route: Oral

Frequency: As per corresponding adult regimen

For treatment and prophylaxis of herpes simplex infections. For neonatal herpes and severe HSV in immunocompromised children, intravenous aciclovir is recommended. Oral aciclovir mainly for non-severe skin and mucosa HSV infections.

Dose adjustments

Renal

Eliminated by renal clearance; dose must be adjusted in renal impairment and in the elderly (reduced renal function). Maintain adequate hydration on high oral doses.

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 or valaciclovir, or to any excipient

Side effects

Headache, dizziness
Nausea, vomiting, diarrhoea, abdominal pain
Pruritus, rashes (including photosensitivity)
Fatigue, fever
Rare: increases in blood urea and creatinine; very rare: neurological effects (confusion, hallucinations, convulsions, encephalopathy), acute renal failure

Interactions

Probenecid and cimetidine increase aciclovir AUC and reduce renal clearance
Increased plasma AUCs of aciclovir and the inactive metabolite of mycophenolate mofetil when co-administered
Concomitant aciclovir increases AUC of administered theophylline by approximately 50% (monitor theophylline levels)

Clinical monograph

How it works

It is activated by viral thymidine kinase and inhibits viral DNA polymerase, selectively suppressing viral DNA replication.

Prescribing in practice

Ensure good hydration and reduce the dose in renal impairment, as it is renally cleared and carries a risk of crystal nephropathy and neurotoxicity, especially intravenously.
It is most effective when started early in the course of infection.
It suppresses active infection but does not eradicate latent virus, so recurrences can still occur.

Monitoring

Monitor renal function with intravenous therapy and in those at risk; watch for neurological effects such as confusion, particularly in renal impairment or the elderly.

Counselling the patient

Drink plenty of fluids during treatment and start the medicine as early as possible.
Report confusion, agitation or reduced urine output, and understand that the medicine controls but does not cure the infection.

Evidence & guidelines

An established first-line antiviral for herpes simplex and varicella-zoster infection, with early initiation improving outcomes, in line with NICE and local guidance.

Reference: NICE CKS (Herpes simplex); TOXBASE; 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