Valaciclovir (Valtrex)
Is It Right for You?
A complete guide to Valaciclovir (Valtrex) — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Valaciclovir (Valtrex)?
Valaciclovir, marketed as Valtrex, is an antiviral prodrug that is the L-valyl ester of aciclovir. After oral administration, it is rapidly and almost completely converted to aciclovir by intestinal and hepatic enzymes, achieving plasma aciclovir concentrations 3–5 times higher than those achievable with equivalent oral aciclovir doses. This superior bioavailability allows for less frequent dosing, significantly improving patient convenience and adherence compared to aciclovir. It is available in tablets of 500 mg and 1000 mg.
Valaciclovir is active against herpes simplex virus (HSV-1 and HSV-2), varicella-zoster virus (VZV), and Epstein-Barr virus (EBV), and is the antiviral of choice for most herpes group virus infections managed in community settings in the UK.
Valaciclovir is a prescription-only medication in the UK. A clinician must assess the clinical diagnosis, severity, renal function, and risk of transmission before prescribing.
What Conditions Is Valaciclovir Used For?
Valaciclovir is indicated for:
year-round nasal and eye allergy symptoms caused by persistent allergens such as house dust mites, pet dander, and mould spores.
Including acute otitis media - especially common in children.
streptococcal pharyngitis and tonsillitis, particularly in patients allergic to penicillin.
Herpes simplex and varicella-zoster viruses establish lifelong latency in sensory nerve ganglia. Valaciclovir suppresses active viral replication and reduces outbreak frequency and severity but does not eradicate latent virus. Asymptomatic viral shedding can still occur during suppressive therapy, meaning transmission risk — though significantly reduced — is not eliminated.
How Does Valaciclovir Work?
Valaciclovir is a prodrug. Following oral absorption, it is cleaved by intestinal and hepatic valaciclovir hydrolase to yield aciclovir and L-valine. The active antiviral compound, aciclovir, then exerts its effect through a highly selective, virus-specific mechanism:
Step 1 — Viral thymidine kinase phosphorylation: Aciclovir is a nucleoside analogue of guanosine. It is selectively monophosphorylated by the viral thymidine kinase (TK) enzyme, which is encoded by herpes viruses but not present in healthy uninfected human cells. This step is critical: it means aciclovir is activated only inside herpes-virus-infected cells, conferring an extremely high therapeutic index and minimal toxicity to uninfected tissue.
Step 2 — Cellular kinase further phosphorylation: Cellular kinases subsequently convert aciclovir monophosphate to the active triphosphate form (aciclovir triphosphate).
Step 3 — Viral DNA polymerase inhibition: Aciclovir triphosphate competitively inhibits the viral DNA polymerase enzyme and acts as a chain terminator when incorporated into replicating viral DNA. Because aciclovir lacks a 3'-hydroxyl group, DNA synthesis cannot continue once it is incorporated. This halts viral DNA replication and prevents new viral particles from forming.
The selectivity for virally infected cells and the requirement for viral TK activation explains why valaciclovir has an excellent safety profile and is safe for long-term suppressive use in otherwise healthy adults.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Start treatment as early as possible. For shingles, valaciclovir is most effective when initiated within 72 hours of rash onset. For genital herpes episodes, starting at the first sign of a prodrome (tingling, itching) maximises efficacy.
Can be taken with or without food. Food does not significantly affect absorption.
Maintain adequate hydration. Especially at higher doses, adequate fluid intake is important to prevent aciclovir crystallisation in renal tubules.
Suppressive therapy is taken continuously. Unlike episodic therapy, suppressive therapy is taken every day regardless of symptom status. Annual clinical review is recommended to reassess the need for continued suppression.
Do not double dose. If a dose is missed, take it as soon as you remember — but skip the missed dose if it is almost time for the next one.
Side Effects of Valaciclovir
Most side effects are mild and resolve on their own. Serious side effects are rare but require immediate medical attention.
- Headache
- Nausea
- Vomiting
- Diarrhoea or abdominal discomfort
- Dizziness
- Thrombotic thrombocytopenic purpura / haemolytic uraemic syndrome (TTP/HUS) --- rare but serious haematological complication, primarily associated with very high doses in immunocompromised patients; presents with microangiopathic haemolytic anaemia, thrombocytopenia, and renal impairment
- Acute kidney injury --- aciclovir crystallises in the renal tubules at high plasma concentrations, particularly in dehydrated patients or those with pre-existing renal impairment; presents with oliguria and rising creatinine
- Neurological toxicity --- confusion, agitation, hallucinations, seizures, and coma; rare at standard doses but risk increases with renal impairment and supratherapeutic dosing; elderly patients are more susceptible
- Severe hypersensitivity reactions --- anaphylaxis, angioedema; rare
Valaciclovir requires dose adjustment in patients with renal impairment (eGFR
Drug Interactions
Valaciclovir may interact with other medications, including nephrotoxic agents and drugs that reduce renal tubular secretion of aciclovir. Always inform your clinician about all medications you are taking.
concurrent use with nephrotoxic drugs (NSAIDs, aminoglycosides, ciclosporin, tacrolimus) increases the risk of acute kidney injury; monitor renal function closely.
these drugs reduce renal tubular secretion of aciclovir, increasing aciclovir plasma levels and the risk of toxicity; use with caution.
mutual interference with renal clearance; increased plasma concentrations of both drugs possible; monitor renal function.
no clinically significant pharmacokinetic interactions with most antivirals at standard doses.
Important Warnings
Valaciclovir must be dose-adjusted in patients with renal impairment. Full-dose valaciclovir in patients with significantly reduced eGFR can cause aciclovir accumulation, leading to crystalline nephropathy, acute kidney injury, and serious neurological toxicity. Always disclose current renal function to your clinician before prescribing.
Adequate oral hydration must be maintained throughout treatment, particularly at higher doses (shingles and CMV prophylaxis regimens), to reduce the risk of aciclovir precipitation in the renal tubules.
New confusion, agitation, tremor, or hallucinations during valaciclovir therapy — particularly in elderly or renally impaired patients — should be treated as a potential sign of aciclovir neurotoxicity. Seek urgent medical review.
Daily suppressive therapy with valaciclovir reduces (but does not eliminate) the risk of transmitting genital herpes to an uninfected partner. It must be used alongside consistent condom use and informed partner communication.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Valaciclovir FAQs
No. Valaciclovir suppresses viral replication and can shorten and reduce the severity of outbreaks, but herpes simplex virus establishes permanent latency in nerve ganglia. Valaciclovir does not eradicate the virus — it manages its activity.
When started at the first sign of an episode (tingling, itching, or prodromal symptoms), valaciclovir typically reduces outbreak duration by 1–2 days and significantly reduces the severity of symptoms. Healing time varies between individuals and is faster in recurrent episodes than in primary first-episode infections.
Daily suppressive valaciclovir reduces asymptomatic viral shedding by approximately 75–80% and halves the risk of transmission to an uninfected partner in clinical trials. However, transmission is not completely prevented — condom use further reduces this risk and is recommended alongside suppressive therapy.
Suppressive therapy is reviewed annually. Many patients experience a reduction in outbreak frequency over time as the immune system adapts to controlling HSV. Annual review allows the clinician and patient to discuss whether ongoing suppression is still beneficial based on recurrence patterns and relationship context.
Valaciclovir achieves significantly higher plasma aciclovir concentrations after oral dosing than aciclovir tablets, due to its superior bioavailability as a prodrug. This means it can be taken less frequently (twice or three times daily rather than five times daily), which significantly improves adherence. Clinical efficacy is equivalent when appropriate doses are used.