Aciclovir prodrug Pregnancy: Use in pregnancy only if the potential benefits outweigh the potential risk; available registry/postmarketing data indicate no malformative or foeto/neonatal toxicity. Use with caution during breastfeeding and only when clinically indicated.

Valaciclovir

Brand names: Valtrex

Valaciclovir is an oral prodrug of aciclovir used to treat herpes simplex infections, herpes zoster (shingles) and to suppress recurrent genital herpes.

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: Herpes zoster (shingles), immunocompetent adults: 1000 mg three times daily for 7 days (3000 mg total daily dose)

Route: Oral

Frequency: Three times daily for 7 days

Start as soon as possible after diagnosis (no data if started more than 72 hours after rash onset). Immunocompromised zoster: 1000 mg three times daily for at least 7 days and for 2 days after crusting of lesions. HSV treatment (immunocompetent adults and adolescents 12 years and over): 500 mg twice daily (3-5 days for recurrent episodes; up to 10 days for initial episodes). Herpes labialis (cold sores): 2000 mg twice in one day, second dose about 12 h (no sooner than 6 h) after the first; do not exceed one day. HSV treatment in immunocompromised adults: 1000 mg twice daily for at least 5 days (up to 10 days for initial episodes). HSV suppression (immunocompetent): 500 mg once daily (or 250 mg twice daily if 10 or more recurrences/year); immunocompromised: 500 mg twice daily. CMV prophylaxis (post-transplant, adults and adolescents 12 years and over): 2000 mg four times daily, usually for 90 days. All doses to be reduced according to creatinine clearance. Children under 12 years: efficacy not evaluated; adolescents 12 years and over use adult HSV doses.

Dose adjustments

Renal

Reduce dose per creatinine clearance (mL/min). Herpes zoster: >=50 -> 1000 mg three times daily; 30-49 -> 1000 mg twice daily; 10-29 -> 1000 mg once daily; <10 -> 500 mg once daily. HSV treatment (immunocompetent): >=30 -> 500 mg twice daily; <30 -> 500 mg once daily. In immunocompromised HSV: >=30 -> 1000 mg twice daily; <30 -> 1000 mg once daily. On intermittent haemodialysis, give the dose after dialysis.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to valaciclovir, aciclovir or any of the excipients

Side effects

Headache (very common)
Nausea
Dizziness
Vomiting, diarrhoea
Rashes including photosensitivity, pruritus
Rare: anaphylaxis; neurological reactions (confusion, hallucinations, encephalopathy) mainly in renal impairment

Clinical monograph

How it works

After absorption it is converted to aciclovir, which viral thymidine kinase phosphorylates to a triphosphate that inhibits viral DNA polymerase and terminates the growing viral DNA chain.

Prescribing in practice

Maintain good hydration and adjust the dose in renal impairment to avoid crystalluria, acute kidney injury and neurotoxicity, which are more likely in the elderly and dehydrated.
Greater oral bioavailability than aciclovir permits a simpler, less frequent dosing schedule.
Treatment is most effective when started promptly after the onset of symptoms.

Monitoring

Check renal function in those with pre-existing renal impairment, advanced age, or concomitant nephrotoxic therapy.

Counselling the patient

Keep well hydrated throughout the course.
Start treatment as soon as symptoms appear, especially for shingles.
Seek advice if you become confused or notice you are passing less urine.

Evidence & guidelines

It is a well-established oral antiviral recommended for herpes simplex and zoster in current prescribing references.

Reference: NICE NG147/CKS; 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.

Pathways