Prodrug Antiviral — HSV / VZV / CMV Prevention (Oral) 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, an antiviral used to treat herpes simplex (including genital herpes) and herpes zoster infections and to suppress recurrent HSV.

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

Infectious-diseases indications from the same SPC. Immunocompromised zoster: 1000 mg three times daily for at least 7 days and for 2 days after crusting of lesions (treat if within one week of vesicle formation or any time before full crusting). HSV treatment: immunocompetent adults/adolescents 12 years and over 500 mg twice daily (recurrent 3-5 days; initial up to 10 days); immunocompromised adults 1000 mg twice daily for at least 5 days (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 suppression: immunocompetent 500 mg once daily (or 250 mg twice daily if 10 or more recurrences/year), immunocompromised 500 mg twice daily; re-evaluate after 6-12 months. CMV prophylaxis (post-transplant): 2000 mg four times daily, initiated as early as possible post-transplant, usually 90 days. All doses reduced according to creatinine clearance. Complicated/visceral/disseminated zoster and immunocompromised ophthalmic zoster: consider intravenous antiviral therapy. Children under 12 years: efficacy not evaluated.

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. HSV (immunocompromised): >=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 or on high CMV-prophylaxis doses

Clinical monograph

How it works

It is rapidly converted to aciclovir, which is phosphorylated by viral thymidine kinase and inhibits viral DNA polymerase, halting viral DNA replication.

Prescribing in practice

Ensure adequate hydration and reduce the dose in renal impairment, as accumulation can cause acute kidney injury and neurotoxicity (confusion, hallucinations), especially in older patients.
Its improved oral bioavailability over aciclovir allows less frequent dosing.
Start treatment as early as possible after symptom onset for best effect, particularly in herpes zoster.

Monitoring

Monitor renal function in patients with impaired kidneys, the elderly, or those on other nephrotoxic drugs.

Counselling the patient

Drink plenty of fluids while taking this medicine.
Begin the course at the first sign of an outbreak.
Report confusion, agitation or reduced urine output.

Evidence & guidelines

Valaciclovir is an established first-line oral agent for herpes simplex and zoster in line with UK and international guidance.

Reference: BASHH Herpes Guidelines 2014; BHIVA HIV Guidelines; Cochrane Review: Antivirals for Herpes Zoster; PHE Shingles Treatment Guidance; 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