Valacyclovir
Antiviral medication for herpes simplex, shingles and cold sores
Quick facts about Valacyclovir
Key takeaways about Valacyclovir
- Prodrug of acyclovir: Valacyclovir is converted to acyclovir in the body, providing about 3–5 times better oral absorption than acyclovir alone
- Multiple herpes infections: Effective against herpes simplex (HSV-1 and HSV-2), varicella-zoster virus (shingles), and cytomegalovirus (CMV)
- Start treatment early: Valacyclovir works best when started within 48–72 hours of symptom onset, especially for shingles and cold sores
- Stay hydrated: Drink plenty of water during treatment, particularly if you are elderly or have kidney problems, to reduce the risk of side effects
- Dose adjustment needed: Patients with kidney impairment require reduced doses – always tell your doctor about any kidney problems
What Is Valacyclovir and What Is It Used For?
Valacyclovir is an antiviral medicine that belongs to a group of drugs called nucleoside analogues. It works by killing or stopping the growth of certain viruses, including herpes simplex virus (HSV), varicella-zoster virus (VZV), and cytomegalovirus (CMV). It is the prodrug form of acyclovir, with significantly improved oral bioavailability.
Valacyclovir is one of the most widely prescribed antiviral medications worldwide. After oral administration, it is rapidly and almost completely converted to acyclovir and the amino acid L-valine by first-pass intestinal and hepatic metabolism. The key advantage of valacyclovir over acyclovir is its substantially higher bioavailability – approximately 55% compared to just 10–20% for oral acyclovir. This means that less frequent dosing is required, making treatment more convenient for patients.
The active metabolite, acyclovir, is selectively taken up by virus-infected cells. Inside these cells, the viral enzyme thymidine kinase phosphorylates acyclovir to acyclovir monophosphate, which is then further phosphorylated by cellular kinases to the active form, acyclovir triphosphate. This active compound inhibits viral DNA polymerase, effectively blocking the replication of herpes viruses. Because acyclovir requires the viral thymidine kinase for activation, it has minimal toxicity to uninfected human cells.
Approved indications
Valacyclovir is approved for use in the following conditions:
- Shingles (herpes zoster): Treatment of herpes zoster in adults. Early initiation of treatment is critical for optimal outcomes
- Genital herpes (HSV): Treatment of initial and recurrent episodes of genital herpes in adults and adolescents over 12 years of age, as well as suppressive therapy to reduce recurrence frequency
- Cold sores (herpes labialis): Treatment of cold sores in adults and adolescents over 12 years of age
- CMV prophylaxis: Prevention of cytomegalovirus infection and disease following solid organ transplantation in adults and adolescents over 12 years
- Recurrent HSV eye infections: Treatment and prevention of recurrent herpes simplex eye infections in adults and adolescents over 12 years
For shingles, treatment should ideally begin within 72 hours of the onset of the rash. For cold sores, the best results are achieved when treatment is started at the earliest sign of a developing sore, such as tingling, itching, or burning. Early treatment can significantly reduce the duration and severity of symptoms.
How valacyclovir differs from acyclovir
Valacyclovir was developed specifically to overcome the poor oral bioavailability of acyclovir. When you take a valacyclovir tablet, your body converts it into acyclovir plus the amino acid valine. The result is blood levels of acyclovir that are 3 to 5 times higher than what you would achieve with an equivalent oral dose of acyclovir itself. This improved absorption allows for simpler dosing regimens – for example, shingles can be treated with valacyclovir three times daily compared to five times daily with acyclovir.
Once converted to acyclovir, the mechanism of action is identical. Both medications ultimately work by inhibiting viral DNA polymerase, blocking herpes virus replication. The safety profile is also comparable, since valacyclovir is essentially a delivery vehicle for acyclovir.
What Should You Know Before Taking Valacyclovir?
Before taking valacyclovir, tell your doctor if you have kidney problems, liver problems, are over 65 years old, have a weakened immune system, are pregnant or breastfeeding, or are taking any other medications. Do not take valacyclovir if you are allergic to valaciclovir or acyclovir.
Like all medicines, valacyclovir requires careful consideration before use. Your doctor will evaluate your individual circumstances to determine whether this medication is appropriate for you and which dose is most suitable. Being open about your medical history and current medications helps ensure safe and effective treatment.
Contraindications
You must not take valacyclovir if:
- You are allergic to valaciclovir or acyclovir, or to any of the other ingredients in the tablets (such as microcrystalline cellulose, crospovidone, povidone, magnesium stearate, hypromellose, titanium dioxide, macrogol 400, polysorbate 80, or carnauba wax)
- You have previously experienced Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) after taking valacyclovir – a condition characterized by widespread skin rash, fever, swollen lymph nodes, elevated liver enzymes, and increased eosinophils in the blood
If you are unsure whether any of the above applies to you, speak with your doctor or pharmacist before starting treatment.
Warnings and Precautions
Talk to your doctor or pharmacist before taking valacyclovir if you have any of the following conditions:
- Kidney problems: Valacyclovir and its active metabolite acyclovir are primarily eliminated by the kidneys. Impaired renal function increases the risk of side effects, particularly neurological symptoms. Your doctor will likely reduce your dose and may monitor your kidney function during treatment
- Liver problems: Although valacyclovir is metabolized in the liver, significant liver impairment could affect drug conversion. Inform your doctor about any liver conditions
- Age over 65: Elderly patients are more likely to have reduced kidney function and may be more susceptible to neurological side effects. Adequate hydration is especially important in this age group
- Weakened immune system: Immunocompromised patients, including those with HIV/AIDS or those taking immunosuppressive medications after organ transplantation, should be monitored carefully. High doses used for CMV prophylaxis carry a risk of thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS)
Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) has been reported with valacyclovir use. Initial symptoms include flu-like illness and facial rash, followed by widespread rash with fever, elevated liver enzymes, increased eosinophils, and swollen lymph nodes.
Stop taking valacyclovir and contact your doctor or seek emergency medical care immediately if you develop a skin rash with fever and swollen lymph nodes.
Preventing transmission of genital herpes
If you are taking valacyclovir to treat or prevent genital herpes, or if you have previously had genital herpes, you should still practise safe sex by using condoms. This is important to help prevent spreading the infection to others. Valacyclovir suppressive therapy has been shown to reduce the risk of transmission, but it does not eliminate it entirely. You should avoid sexual contact when you have active genital sores or blisters.
Pregnancy and Breastfeeding
Valacyclovir is not routinely recommended for use during pregnancy. If you are pregnant, think you may be pregnant, or are planning to become pregnant, do not take valacyclovir without first consulting your doctor. Your doctor will weigh the benefits to you against the potential risks to your baby.
Limited data from pregnancy registries have not indicated an increased risk of birth defects among women exposed to acyclovir (the active metabolite of valacyclovir) during the first trimester. However, the number of pregnancies studied is insufficient to draw definitive conclusions. Animal reproduction studies have not demonstrated direct harmful effects on fertility or fetal development.
Acyclovir, the active metabolite, is excreted in breast milk. Following oral administration of valacyclovir 500 mg, peak acyclovir concentrations in breast milk range from 0.5 to 2.3 times the corresponding plasma concentrations. The decision to continue breastfeeding during valacyclovir treatment should be made in consultation with your healthcare provider, weighing the benefits of breastfeeding against the potential risk to the infant.
Driving and operating machinery
Valacyclovir may cause side effects that could affect your ability to drive or operate machinery. Dizziness, drowsiness, and confusion have been reported, although they are uncommon. Do not drive or operate machinery until you are sure you are not affected by the medication. This is particularly important when starting treatment, as it may take a few days to understand how the medicine affects you.
How Does Valacyclovir Interact with Other Drugs?
Valacyclovir can interact with medications that affect kidney function, including aminoglycosides, platinum-based chemotherapy agents, iodinated contrast media, methotrexate, pentamidine, foscarnet, ciclosporin, tacrolimus, cimetidine, and probenecid. Always tell your doctor about all medications you are taking.
Drug interactions with valacyclovir are primarily related to its renal elimination. Since both valacyclovir and its active metabolite acyclovir are excreted through the kidneys, any medication that affects renal function or competes for renal tubular secretion can potentially alter drug levels and increase the risk of side effects. This is especially relevant for elderly patients, those with pre-existing kidney impairment, and transplant recipients taking multiple medications.
It is important to inform your doctor or pharmacist about all medicines you are currently taking, including prescription medications, over-the-counter drugs, herbal remedies, and supplements. This is particularly critical if you are taking valacyclovir for shingles treatment or following organ transplantation, where higher doses are typically prescribed.
| Interacting Drug | Effect | Severity | Clinical Advice |
|---|---|---|---|
| Cimetidine / Probenecid | Reduce renal clearance of acyclovir, increasing blood levels | Moderate | Monitor for increased side effects; dose adjustment may be needed |
| Aminoglycosides | Additive nephrotoxicity; combined kidney damage risk | Major | Avoid combination if possible; monitor renal function closely |
| Ciclosporin / Tacrolimus | Additive nephrotoxicity; increased risk of renal impairment | Major | Monitor renal function and drug levels frequently |
| Methotrexate | Potential competition for renal tubular secretion | Moderate | Monitor for methotrexate toxicity; check blood counts |
| Mycophenolate mofetil | Competition for renal tubular secretion; may increase levels of both drugs | Moderate | Monitor for side effects of both medications |
| Platinum compounds (cisplatin) | Additive nephrotoxicity | Major | Ensure adequate hydration; monitor renal function |
| Foscarnet | Additive nephrotoxicity | Major | Avoid concurrent use; monitor renal function if unavoidable |
If you have received an organ transplant, you are likely taking several medications that can interact with valacyclovir. Your transplant team will carefully coordinate your medications and monitor your kidney function. Always inform all healthcare providers about your complete medication list.
What Is the Correct Dosage of Valacyclovir?
Valacyclovir dosage varies by condition: shingles 1000 mg three times daily for 7 days; cold sores 2000 mg twice in one day; genital herpes 500 mg twice daily for 3–10 days; suppressive therapy 500 mg once daily; CMV prophylaxis 2000 mg four times daily for approximately 3 months. Always follow your doctor's instructions.
Always take valacyclovir exactly as your doctor has told you. Do not change your dose or stop taking the medication without consulting your healthcare provider. The correct dosage depends on the condition being treated, and your doctor may adjust the dose based on factors such as your age, kidney function, and immune status.
| Condition | Dose | Frequency | Duration |
|---|---|---|---|
| Shingles (herpes zoster) | 1000 mg | Three times daily | 7 days |
| Cold sores (herpes labialis) | 2000 mg | Twice daily (12 hours apart, not less than 6 hours) | 1 day only (2 doses) |
| Genital herpes – first episode | 500 mg | Twice daily | 5–10 days |
| Genital herpes – recurrence | 500 mg | Twice daily | 3–5 days |
| Genital herpes – suppressive therapy | 500 mg | Once daily (or 250 mg twice daily for frequent recurrences) | As directed by doctor |
| CMV prophylaxis (post-transplant) | 2000 mg | Four times daily (every 6 hours) | Approx. 3 months |
How to take this medicine
- Take the tablets by mouth, swallowed whole with water
- Valacyclovir can be taken with or without food
- Try to take your doses at the same times each day to maintain steady drug levels
- Follow your doctor's or pharmacist's instructions carefully
Special considerations for elderly patients and those with kidney problems
If you are over 65 years of age or have kidney problems, it is very important that you drink water regularly throughout the day while taking valacyclovir. Adequate hydration helps reduce the risk of side effects that can affect the kidneys or nervous system. Your doctor will monitor you for signs of these problems and may prescribe a lower dose based on your kidney function.
Your doctor may adjust your dose if:
- You are older than 65 years
- You have a weakened immune system
- You have kidney problems (your doctor will calculate the dose based on your creatinine clearance)
Missed Dose
If you forget to take a dose of valacyclovir, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a forgotten dose.
Overdose
Valacyclovir is generally not harmful if you accidentally take slightly more than prescribed. However, if you take too many tablets, you may experience nausea, vomiting, kidney problems, confusion, agitation, changes in consciousness, hallucinations, or loss of consciousness. If you suspect an overdose, contact your doctor, hospital, or poison control centre immediately. Take the medicine packaging with you to help the healthcare team identify what you have taken.
Acyclovir is removed by haemodialysis, so haemodialysis may be considered in cases of severe overdose, particularly in patients with kidney failure.
What Are the Side Effects of Valacyclovir?
The most common side effect of valacyclovir is headache (affecting more than 1 in 10 users). Common side effects include nausea, dizziness, vomiting, diarrhea, skin rash, itching, and photosensitivity. Serious but rare side effects include kidney problems, neurological symptoms (confusion, hallucinations, seizures), and severe allergic reactions.
Like all medicines, valacyclovir can cause side effects, although not everyone will experience them. Most side effects are mild and temporary, resolving once treatment is completed or the dose is adjusted. However, some side effects require immediate medical attention.
- Severe allergic reactions (anaphylaxis): Rapid onset of rash, itching, swelling of lips, face, neck and throat causing breathing difficulty, or a drop in blood pressure leading to collapse
- Skin rash or redness: Skin reactions with or without blisters, skin irritation, swelling (DRESS syndrome), fever and flu-like symptoms
Very Common
- Headache
Common
- Nausea
- Dizziness
- Vomiting
- Diarrhea
- Skin reactions after sun exposure (photosensitivity)
- Skin rash
- Itching (pruritus)
Uncommon
- Confusion
- Hallucinations (seeing or hearing things that are not there)
- Drowsiness
- Tremor (shaking)
- Agitation
- Shortness of breath (dyspnoea)
- Stomach discomfort
- Skin rash, sometimes itchy; hives (urticaria)
- Lower back pain (kidney pain)
- Blood in urine (haematuria)
- Decreased white blood cell count (leucopenia)
- Decreased platelet count (thrombocytopenia)
- Elevated liver enzymes
Rare
- Unsteady gait or lack of coordination (ataxia)
- Slow and slurred speech (dysarthria)
- Seizures (convulsions)
- Altered brain function (encephalopathy)
- Loss of consciousness (coma)
- Confused or disturbed thoughts (delirium)
- Kidney problems with reduced or absent urine output
Frequency Not Known
- DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms): widespread rash, fever, elevated liver enzymes, abnormal blood counts, swollen lymph nodes
- Kidney inflammation (tubulointerstitial nephritis)
Uncommon and rare neurological side effects (confusion, hallucinations, drowsiness, tremor, agitation, seizures, encephalopathy, coma, and delirium) typically occur in patients with kidney problems, elderly patients, or organ transplant recipients taking high doses of valacyclovir (8 g or more per day). These symptoms usually improve when valacyclovir is discontinued or when the dose is reduced. If you experience any neurological symptoms, contact your healthcare provider promptly.
How Should You Store Valacyclovir?
Store valacyclovir at room temperature, below 30°C (86°F), in the original packaging. Keep out of sight and reach of children. Do not use after the expiry date printed on the packaging. Dispose of unused medication properly – do not flush down the toilet or put in household waste.
Proper storage of medications is essential to maintain their effectiveness and safety. Valacyclovir tablets should be stored under the following conditions:
- Temperature: Store below 30°C (86°F). Do not refrigerate or freeze
- Packaging: Keep the tablets in their original blister packaging to protect from moisture and light
- Children: Store the medicine out of the sight and reach of children
- Expiry date: Do not use the medication after the expiry date (EXP) printed on the carton. The expiry date refers to the last day of that month
- Disposal: Do not throw medicines down the drain or in household waste. Ask your pharmacist about how to dispose of medicines that are no longer needed. These measures help protect the environment
If you notice any visible changes in the tablets, such as discolouration or crumbling, do not take them and consult your pharmacist.
What Does Valacyclovir Contain?
Each valacyclovir tablet contains the active substance valaciclovir (as valaciclovir hydrochloride) in strengths of 250 mg, 500 mg, or 1000 mg. Inactive ingredients include microcrystalline cellulose, crospovidone, povidone, magnesium stearate, and a film coating of hypromellose, titanium dioxide, macrogol 400, polysorbate 80, and carnauba wax.
Active ingredient
The active substance is valaciclovir (present as valaciclovir hydrochloride). Tablets are available in three strengths:
- 250 mg valaciclovir per tablet
- 500 mg valaciclovir per tablet
- 1000 mg valaciclovir per tablet
Inactive ingredients (excipients)
The other ingredients are:
Tablet core:
- Microcrystalline cellulose
- Crospovidone
- Povidone
- Magnesium stearate
- Colloidal anhydrous silica
Film coating:
- Hypromellose
- Titanium dioxide
- Macrogol 400
- Polysorbate 80 (500 mg and 1000 mg tablets only)
- Carnauba wax
Appearance and packaging
Valacyclovir tablets are white, film-coated, and supplied in PVC/aluminium foil blister packs. Pack sizes vary by strength and manufacturer but commonly include:
- 250 mg tablets: Packs of 60 tablets
- 500 mg tablets: Packs of 10, 30, 42, or 112 tablets
- 1000 mg tablets: Packs of 21 tablets
Not all pack sizes may be available in all markets. Different manufacturers may have slightly different packaging configurations.
Frequently Asked Questions About Valacyclovir
Medical References and Sources
This article is based on current medical research, official prescribing information, and international guidelines. All claims are supported by scientific evidence from peer-reviewed sources.
- European Medicines Agency (EMA). "Valaciclovir – Summary of Product Characteristics (SmPC)." EMA Official European product information for valaciclovir-containing medicines.
- U.S. Food and Drug Administration (FDA). "Valtrex (valacyclovir hydrochloride) – Prescribing Information." FDA Prescribing Information Complete FDA-approved prescribing information for Valtrex.
- World Health Organization (WHO). "WHO Model List of Essential Medicines – 23rd list (2023)." WHO Essential Medicines Acyclovir is listed as an essential medicine by WHO for the treatment of herpes simplex infections.
- British National Formulary (BNF). "Valaciclovir – Drug Monograph." BNF Online Comprehensive drug information including indications, dosage, side effects, and interactions.
- Tyring SK, et al. (2012). "Valacyclovir for herpes simplex virus infection: long-term safety and sustained efficacy after 20 years' experience with acyclovir." Journal of Infectious Diseases. 186(Suppl 1):S40–6. Comprehensive review of long-term safety data for valacyclovir therapy.
- Weller S, et al. (1993). "Pharmacokinetics of the acyclovir pro-drug valaciclovir after escalating single- and multiple-dose administration to normal volunteers." Clinical Pharmacology & Therapeutics. 54(6):595–605. doi:10.1038/clpt.1993.196 Foundational pharmacokinetic study demonstrating improved bioavailability of valacyclovir over acyclovir.
- Corey L, et al. (2004). "Once-daily valacyclovir to reduce the risk of transmission of genital herpes." New England Journal of Medicine. 350(1):11–20. doi:10.1056/NEJMoa035144 Landmark clinical trial demonstrating valacyclovir's effectiveness in reducing herpes transmission.
Evidence grading: This article uses the GRADE framework (Grading of Recommendations Assessment, Development and Evaluation) for evidence-based medicine. Drug information is cross-referenced against multiple authoritative sources including EMA SmPC, FDA prescribing information, BNF, and WHO guidelines.
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