Antiviral (purine nucleoside analogue) Pregnancy: Use only when potential benefits outweigh possible unknown risks; post-marketing registry has not shown an increase in birth defects, but caution advised (UK SPC §4.6)

Aciclovir

Brand names: Zovirax

Aciclovir is a guanosine analogue antiviral used to treat and suppress herpes simplex and varicella zoster virus 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); in severely immunocompromised patients or impaired gut absorption the dose can be doubled to 400 mg, or IV dosing considered. Suppression of herpes simplex (immunocompetent): 200 mg four times daily (six-hourly), or 400 mg twice daily; titrate down to 200 mg three times daily or twice daily; interrupt every 6-12 months. Prophylaxis of herpes simplex (immunocompromised): 200 mg four times daily. Treatment of varicella and herpes zoster: 800 mg five times daily at four-hourly intervals (omitting night-time dose) for 7 days. Begin dosing as early as possible after onset. Paediatric: Treatment and prophylaxis of herpes simplex: children aged two years and over should be given adult dosages; children below two years should be given half the adult dose. For neonatal herpes virus infections, intravenous aciclovir is recommended.

Dose adjustments

Renal

Aciclovir is eliminated by renal clearance; the dose must be adjusted in patients with renal impairment, and elderly patients (likely reduced renal function) may need dose adjustment.

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 of the excipients

Side effects

Headache, dizziness (common)
Nausea, vomiting, diarrhoea, abdominal pain (common)
Pruritus, rashes including photosensitivity (common)
Fatigue, fever (common)
Increases in blood urea and creatinine (rare); acute renal failure, renal pain (very rare)

Interactions

Probenecid and cimetidine increase the AUC of aciclovir and reduce its renal clearance
Mycophenolate mofetil (increased plasma AUCs of aciclovir and of the inactive mycophenolate metabolite when co-administered)
Theophylline (concomitant aciclovir increases theophylline AUC by approximately 50%; measure plasma concentrations)

Clinical monograph

How it works

It is selectively phosphorylated by viral thymidine kinase to its active triphosphate, which inhibits viral DNA polymerase and terminates the growing viral DNA chain, sparing uninfected cells.

Prescribing in practice

Maintain adequate hydration and adjust dosing in renal impairment, as aciclovir can crystallise in renal tubules and cause acute kidney injury, particularly with intravenous use or in dehydrated or elderly patients.
Intravenous infusions should be given slowly over the recommended period rather than as a bolus to reduce the risk of nephrotoxicity.
Neurological effects such as confusion or agitation can occur, especially with renal impairment, and warrant dose review.

Monitoring

Monitor renal function and hydration during intravenous and high-dose treatment, particularly in older or renally impaired patients.

Counselling the patient

Drink plenty of fluids while taking aciclovir, especially at higher doses.
Start treatment as early as possible for the best effect on herpes and shingles episodes.
Report confusion, drowsiness or reduced urine output to your clinician.

Evidence & guidelines

Aciclovir is the long-established first-line treatment for herpes simplex and varicella zoster infections in UK practice, supported by extensive clinical experience and national guidance.

Reference: BASHH guidelines; NICE CG105; 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