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What is valacyclovir?

Valacyclovir is an antiviral drug that belongs to the purine (guanine) nucleoside analog drug class.¹

As a prodrug to acyclovir (Zovirax), it rapidly converts to acyclovir when absorbed in the body. Through a series of biochemical reactions, the drug becomes a compound that inhibits viruses from reproducing.² ³

Valacyclovir does not eradicate the viruses that cause the conditions for which it is indicated, but it slows their growth. In this way, it helps your body fight the infection, improves symptoms, and promotes faster healing.⁴

What is valacyclovir used to treat?

Valacyclovir has been approved by the US Food and Drug Administration (FDA) for the treatment of the following viral infections:⁵

Herpes simplex virus (HSV)

The drug is prescribed to treat the following conditions caused by the herpes simplex virus:

  • Genital herpes (adults)

  • Oral herpes (adults and children over 12 years of age)

Patients are treated with a course of valacyclovir for the first episode of herpes infection. The virus remains in the body and recurrences can be expected, although they are typically less severe. A lower dose of the drug can be given for a shorter period when recurrent episodes occur, reducing their severity and duration if started promptly upon noting the first symptoms.

You might take the medication on a daily basis if you have frequent recurrences. This is called suppression therapy. It helps reduce the frequency and severity of recurrent episodes.

The virus remains in your body even after you have recovered from oral or genital herpes, and you can still infect other people while taking valacyclovir. However, taking the drug reduces the risk of genital herpes transmission by almost 50%.⁶

Valacyclovir is also used off-label for treating genital herpes in preadolescents and adolescents, but this is not an FDA-approved use.


Valacyclovir is indicated for the treatment of shingles (herpes zoster) in adults.

Shingles develops as a recurrent infection from the dormant chickenpox virus, usually occurring in older adults or people who are immunocompromised.


Children aged 2–18 years with active chickenpox may be prescribed valacyclovir to relieve symptoms.

Dosage forms and strengths

Valacyclovir comes as an oral tablet in strengths of 500mg and 1,000mg, both as a generic and the brand Valtrex.⁷

Your doctor will prescribe a dosage for you depending on the condition you are being treated for, your age, and the frequency of your symptoms.

There is no liquid form of the medication formulated for purchase. However, if you are unable to swallow pills, some pharmacies can prepare an oral suspension with a doctor’s prescription.

How do you take valacyclovir?

Take valacyclovir precisely as directed by the prescribing medical professional. Read the prescription label carefully before use.

If you have recurrent herpes, your doctor will prescribe extra medication to keep on hand for treating future outbreaks. For optimal results, take valacyclovir as soon as possible after you detect the first herpes symptoms, such as blisters, burning, or tingling. The drug will be much more effective if you start taking it within 1–2 days of symptom onset.

You can take valacyclovir with or without food.

This medication is filtered by the kidneys and can cause damage in some cases called acute kidney injury (AKI). It’s best to drink plenty of water to stay well hydrated while taking valacyclovir.⁹ ¹⁰

Do not stop taking valacyclovir even after your symptoms start to clear. Continue taking the medication as prescribed for the full length of treatment.¹¹

Seeing results

The sooner you start taking the drug after symptoms appear, the more effective it will be. Improvement in symptoms can be seen within days of beginning a course of valacyclovir.¹²

The length of your prescription will depend on the condition being treated. Your doctor may prescribe a longer course of treatment if your symptoms have not improved or to reduce the possibility of a recurrent episode.

Who should not take valacyclovir?

Valacyclovir is not suitable for everyone. You should not take this drug if you have a history of an allergic reaction to valacyclovir, acyclovir, or any of their inactive ingredients. You can find a full list of ingredients in the leaflet that comes with your medication.

Valacyclovir should be used with caution in people with any of the following conditions:

  • History of a kidney or bone marrow transplant

  • Advanced HIV infection or acquired immune deficiency syndrome (AIDS)

  • Immunocompromised or taking immunosuppressants

  • Kidney disease

  • Pregnant or planning a pregnancy

  • Breastfeeding or planning to breastfeed

Potential side effects of valacyclovir

Valacyclovir causes side effects in some people. Possible side effects range from mild to severe.

Common side effects

The most common side effects reported by those taking valacyclovir include any of the following:¹³

  • Headache

  • Neutropenia (low levels of neutrophils in the blood)

  • High blood levels of aspartate transaminase (AST), alanine transaminase (ALT), or alkaline phosphatase (ALP)

  • Nasopharyngitis (the common cold)

  • Nausea

  • Diarrhea

  • Abdominal pain

Less common side effects

The following side effects occur less often in people who take valacyclovir:¹⁴

  • Menstrual pain

  • Depression, fatigue

  • Arthralgias (joint stiffness)

  • Vomiting

  • Dizziness

  • Rash

  • Rhinorrhea (runny nose)

  • Thrombocytopenia (low levels of blood platelets)

  • Leukopenia (low levels of white blood cells)

Rare side effects

The following rare side effects have been reported by people taking this medication:¹⁵

  • Agitation, aggressive behavior, confusion, delirium, psychosis

  • Seizures

  • Aplastic anemia

  • AKI

  • Erythema multiforme (a skin disorder)

  • Hypertension (high blood pressure)

  • Tachycardia (fast heart rate)

  • Tremor

  • Visual disturbances

This is not an exhaustive list of possible valacyclovir side effects. If you or your child experiences any of these or any other unusual symptoms, seek emergency medical attention immediately.


Taking too much valacyclovir can increase your risk of severe adverse effects. It can even be fatal in some cases.

If you suspect that you or someone you know has overdosed, call the National Poison Control helpline at 1-800-222-1222.

Valacyclovir overdose can cause collapse, seizure, trouble breathing, hallucination, confusion, and extreme tiredness. Call 911 if these symptoms occur.¹⁶ ¹⁷

Allergy information

If you are allergic to either valacyclovir or acyclovir, you shouldn’t take this drug. Keep in mind that the medication may also contain other non-active ingredients you might be allergic to. Discuss your allergies with your doctor before taking valacyclovir.

The following are signs of an allergic reaction to this drug:¹⁸

  • Anaphylaxis

  • Swelling of the face, tongue, or throat

  • Trouble breathing

  • Rapid heart rate

  • Rash, hives, itchy skin

  • Vomiting

  • Loss of consciousness

If you experience any of these symptoms, seek medical attention. If your symptoms are severe, call 911 to request immediate medical help.

Long-term use of valacyclovir

Clinical trials had previously found that acyclovir was safe for long-term use in patients who took it for up to 10 years to suppress HSV infection. Subsequent one-year studies on valacyclovir suppression therapy have also demonstrated safety and no significant differences in side effects from treatment with acyclovir.¹⁹

Pregnancy category

Valacyclovir was designated by the FDA as a pregnancy category B medication. This means studies in animals did not demonstrate a risk to the fetus, but there are no adequate, well-controlled human studies to assess whether the drug is safe for use during pregnancy.²

Valacyclovir and pregnancy

Valacyclovir has not been found to increase the risk of miscarriage or birth defects.²¹

It is unclear how valacyclovir affects late-stage pregnancy in relation to problems such as preterm delivery and low birth weight. However, the risk of an untreated viral infection in the mother is high as it can cause problems in the newborn.

Your doctor will tell you whether it’s safe to use valacyclovir during pregnancy. They will assess the benefits versus any potential risks of taking the drug.

The risks of HSV infection during pregnancy

Developing genital herpes from an HSV infection during pregnancy has been linked to miscarriage, fetal growth restriction, preterm labor, and herpes infection in the infant.

If you have an active case of genital herpes, you can transmit it to your baby during delivery. This mode of transmission accounts for 85–90% of cases in newborns.²²

Transmission is more likely to occur upon your first herpes outbreak, as your immune system has not yet had a chance to develop sufficient antibodies to keep the virus in check before delivery. Your doctor will likely prescribe valacyclovir or another antiviral drug to help prevent transmission, or they will recommend cesarean delivery.

Newborns are more likely to contract an HSV infection when there are maternal lesions present during delivery. However, in many cases of neonatal transmission, mothers with an HSV infection do not show symptoms at the time of delivery (and some don’t even know they are infected). Therefore, if you have any known history of genital herpes, your doctor may recommend taking valacyclovir before your delivery to prevent a recurrence and reduce the risk of transmission to your newborn.²³

Neonatal HSV infection can be very serious, causing death or long-term disabilities in some cases. In 60% of cases, symptoms appear five days after birth, but in others, they don’t appear until 4–6 weeks after birth.²⁴

Valacyclovir and breastfeeding

Acyclovir can be found in human breast milk after valacyclovir administration at very low levels. The drug is not believed to cause any adverse effects in breastfed infants.²⁵

Missed doses

It’s important to take valacyclovir as directed by your doctor. If you forget to take your scheduled dose for any reason, take it as soon as you remember. If it’s almost time for your next dose, skip the missed one and continue with your regular dosing schedule. Don’t take two doses to make up for the one you missed.²⁶

Drug interactions

Some drugs can alter how valacyclovir works and reduce or change its effectiveness. Similarly, taking valacyclovir with other substances can cause unexpected reactions and harmful side effects.

Before starting on valacyclovir, tell your doctor about all medications, whether over-the-counter (OTC) or prescription, along with any herbs, supplements, and substances you take regularly or occasionally.

Below are some of the drugs that are known to interact with valacyclovir.

  • Acyclovir: Taking the two drugs together may cause an overdose since valacyclovir is converted into acyclovir when absorbed into your body. The same effects can occur with other antiviral medications, such as valganciclovir (Valcyte), ganciclovir, and cidofovir, and those used to treat HIV infection like zidovudine.²⁷

  • Antibiotics, such as streptomycin, amikacin, gentamicin, tobramycin, clindamycin, and polymyxin B: When these medications are taken with valacyclovir, levels of both drugs may increase in the body. Kidney toxicity, hearing loss, and other adverse reactions are some of the possible effects.

  • Chemotherapy drugs, such as axitinib (Inlyta), bevacizumab (Alymsys), bleomycin, pazopanib (Votrient), cisplatin, and others: Levels of these drugs may be elevated when taken alongside valacyclovir, leading to toxic effects.

  • Non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibuprofen (Motrin, Advil), celecoxib (Celebrex), diclofenac (Cambia, Zipsor, Zorvolex), etodolac, ketorolac, meloxicam (Mobic), and piroxicam (Feldene): Taking valacyclovir alongside an NSAID could increase your risk of AKI.²⁸

  • Cimetidine (Tagamet), an H₂ blocker used to reduce the amount of acid in the stomach: This drug may interact with valacyclovir to reduce its conversion rate to acyclovir. It may also reduce the kidneys’ ability to clear acyclovir from the body.²⁹

  • Probenecid, used in the treatment of gout: This drug may reduce the conversion rate of valacyclovir to acyclovir, lower acyclovir’s clearance rate from the kidneys, and extend its half-life. This may raise the levels of acyclovir in your body, increasing your risk of adverse effects.³⁰

  • Fezolinetant (Veozah), a drug that treats the symptoms of menopause: Valacyclovir can cause increased levels of this drug, raising the risk of adverse effects.

  • Cyclosporine (Gengraf, Neoral, Sandimmune), an immunosuppressive (anti-rejection) drug: This medication may interact with acyclovir to increase your risk of toxic effects on the kidneys. This can also occur with other immunosuppressants, such as methotrexate (Trexall) and everolimus (Zortress).

  • Amphotericin B — an antifungal drug: This may interact with acyclovir to increase your risk of kidney toxicity.

This is not a complete list of drug interactions. Be sure to review your medication list with your doctor before taking valacyclovir.

Can I drink alcohol while taking valacyclovir?

The effects of drinking alcohol while taking valacyclovir are unknown. It’s best to ask your doctor for guidance.

What to discuss with your doctor before starting valacyclovir

Before starting valacyclovir treatment, discuss any concerns and risk factors with your doctor. Be sure to mention any of the following that apply to you:

  • Your medical history and current medical conditions — tell your doctor about any kidney problems you have, immune problems, HIV, or history of organ transplant³¹

  • Pregnancy or plans to become pregnant

  • Breastfeeding or plans to breastfeed

  • History of allergic reactions to valacyclovir, acyclovir, or any of the inactive ingredients in these medicines

  • All other prescription and OTC drugs, vitamins, supplements, or herbal products you are currently taking, take occasionally, or plan to start taking

  • Valacyclovir’s potential side effects

  • What to do if you miss a dose or take too much of the drug

Stopping valacyclovir

If you suddenly stop taking valacyclovir without finishing the prescribed dose, your symptoms may continue, return, or worsen.

Don’t stop taking your medicine until you have taken the full course prescribed for you, you have a severe adverse reaction, or your doctor tells you to stop taking it.

Drug approval history

Valacyclovir was first approved by the FDA in 1995 to be sold as Valtrex, a brand-name drug. Currently, both Valtrex and the generic formulation are available in the US by prescription only.³²

Tips for taking valacyclovir

  • Take valacyclovir exactly as prescribed by your doctor.

  • Do not stop taking your medication until your prescription ends or your doctor advises you to stop.

  • Stay well hydrated when taking the drug to help prevent adverse effects on your kidneys.

  • Avoid sexual contact if you’re taking valacyclovir for genital herpes as this may cause the virus to spread to your partner. You may use latex condoms to help prevent the spread.

  • Store valacyclovir tablets at 20–25ºC (68–77ºF) in an airtight container out of reach of children.³³

  1. Valacyclovir l NIH: PubChem

  2. Valacyclovir l NIH: LiverTox: Clinical and Research Information on Drug-Induced Liver Injury

  3. LABEL: VALACYCLOVIR HYDROCHLORIDE tablet, film coated (2023) l NIH: DailyMed

  4. Valacyclovir l MedlinePlus

  5. LABEL: VALACYCLOVIR HYDROCHLORIDE tablet, film coated (2023) l NIH: DailyMed

  6. Standard-dose and high-dose daily antiviral therapy for short episodes of genital HSV-2 reactivation: three randomised, open-label, cross-over trials (2012)

  7. Valacyclovir l NIH: LiverTox: Clinical and Research Information on Drug-Induced Liver Injury

  8. High-Dose, Short-Duration, Early Valacyclovir Therapy for Episodic Treatment of Cold Sores: Results of Two Randomized, Placebo-Controlled, Multicenter Studies (2003)

  9. Acute renal injury induced by valacyclovir hydrochloride: A case report (2016)

  10. LABEL: VALACYCLOVIR HYDROCHLORIDE tablet, film coated (2023) l NIH: DailyMed

  11. Valacyclovir l MedlinePlus

  12. High-Dose, Short-Duration, Early Valacyclovir Therapy for Episodic Treatment of Cold Sores: Results of Two Randomized, Placebo-Controlled, Multicenter Studies (2003)

  13. Valtrex (valacyclovir) dosing, indications, interactions, adverse effects, and more l Reference Medscape

  14. (As above)

  15. (As above)

  16. Valacyclovir l MedlinePlus

  17. A Unique Case of Valacyclovir Toxicity and Pseudobulbar Affect in a Patient On Peritoneal Dialysis (2021)

  18. LABEL: VALACYCLOVIR HYDROCHLORIDE tablet, film coated (2023) l NIH: DailyMed

  19. Valacyclovir for Herpes Simplex Virus Infection: Long-Term Safety and Sustained Efficacy after 20 Years’ Experience with Acyclovir (2002)

  20. Acyclovir Safe for Treating Herpes Infection in Early Pregnancy (2011)

  21. Acyclovir (Zovirax®) / Valacyclovir (Valtrex®) l NIH: Mother To Baby

  22. Herpes Simplex Virus Infection in Pregnancy (2012)

  23. Neonatal Herpes Simplex Virus (HSV) Infection (2022) l MSD Manuals

  24. Herpes Simplex Virus Infection in Pregnancy (2012)

  25. LABEL: VALACYCLOVIR HYDROCHLORIDE tablet, film coated (2023) l NIH: DailyMed

  26. (As above)

  27. Guidelines for the Prevention and Treatment of Opportunistic Infections in Children with and Exposed to HIV — Table 5. Significant Drug Interactions for Drugs Used to Treat or Prevent Opportunistic Infections (2013) | NIH: HIV Info

  28. Association between Concomitant Use of Acyclovir or Valacyclovir with NSAIDs and an Increased Risk of Acute Kidney Injury: Data Mining of FDA Adverse Event Reporting System (2018)

  29. Multiple Interactions of Cimetidine and Probenecid with Valaciclovir and Its Metabolite Acyclovir (2002)

  30. (As above)

  31. Concurrent Nephrotoxicity and Neurotoxicity Induced by Oral Valacyclovir in a Patient With Previously Normal Kidney Function (2022)

  32. Drug approval package (Valtrex) | US Food and Drug Administration

  33. LABEL: VALACYCLOVIR HYDROCHLORIDE tablet, film coated (2023) l NIH: DailyMed

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.