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Valacyclovir 101: How it Works, Side Effects & More

Dr. Patrick Carroll, MD

Reviewed by Patrick Carroll, MD

Written by Our Editorial Team

Published 05/15/2018

Updated 02/24/2020

Valacyclovir is an antiviral medication that’s used to treat herpes outbreaks and cold sores. It’s sold under several brand names, the most common of which is Valtrex®.

Valacyclovir is one of the most commonly used viral  treatments. It’s used to manage outbreaks of both of the herpes simplex viruses, HSV-1 (cold sores) and HSV-2 (genital herpes). It’s also used in the management of shingles, or herpes zoster.

If you have herpes, it’s extremely likely that you’ll be prescribed either valacyclovir or a similar drug in order to manage the virus. Valacyclovir is highly effective in managing herpes. It’s also an extremely safe, well studied medication.

In this guide, we’ll go over what valacyclovir is, how it works and the potential side effects you can expect from using it as part of herpes management. We’ll also look at how long it can take for valacyclovir to become effective, as well as changes you may need to make to your habits.

What is Valacyclovir?

Valacyclovir is an antiviral medication that’s designed to manage  HSV (herpes simplex virus) and VZV (varicella-zoster virus, more commonly known as chickenpox) infections.

The drug works by slowing down the growth of herpes, making it easier for the body to control the infection. Valacyclovir itself is a prodrug (meaning it converts into another substance in the body), that converts into acyclovir after it passes through the liver.

Compared to acyclovir, which is also used as an antiviral herpes medication, valacyclovir has a far higher level of bioavailability, meaning more of the drug will make it into your body than other herpes drugs that are broken down by the liver.

Valacyclovir treats both symptoms and outbreaks of several species of the herpes virus, ranging from HSV-1 and HSV-2 to VZV (chickenpox), the Epstein–Barr virus (known as HHV-4, which is associated with some forms of cancer) and cytomegalovirus (CMV).

Of these viruses, valacyclovir is most effective against HSV, making it one of the most widely prescribed drugs for people infected with the herpes virus.

Valacyclovir is an extremely well studied medication, with a huge amount of scientific evidence to back up its efficacy and safety record. It’s used to treat people of all ages, from children that suffer from chickenpox to adults with cold sores and recurrent genital herpes.

Since it was introduced in the 1980s, valacyclovir has been widely used to successfully control and treat the symptoms of herpes, so much so that it and similar drugs are listed as the “gold standard” in herpes treatment in studies.

How Valacyclovir (Valtrex) Works

Valacyclovir works through a complex chemical process that interferes with the mechanism the herpes virus uses to reproduce, preventing it from multiplying and reducing the rate at which the infection spreads.

In short, valacyclovir stops the herpes virus from spreading to healthy cells, reducing the effects of the virus.

It’s important to know that valacyclovir doesn’t cure herpes. However, it does make the effects of herpes less severe, meaning the sores developed from the virus heal faster and the aches, cold symptoms and other signs of herpes can become less severe.

If you have herpes, valacyclovir reduces the risk of you transmitting the virus to other people. In one study, researchers found that people with HSV-2 using valacyclovir were almost 50 percent less likely to transfer the virus to their sexual partners than people with HSV-2 that didn’t use medication.

How Long Does Valacyclovir Take to Work?

The amount of time required for valacyclovir to control a herpes or chickenpox outbreak varies based on the severity of the outbreak and how soon you take valacyclovir after noticing herpes symptoms.

For most initial herpes outbreaks and cases of recurring herpes, valacyclovir takes effect very quickly and provides some level of relief in as little as two to three days. Generally, the sooner you take valacyclovir after noticing symptoms, the faster it will be to provide relief.

It’s important to remember that every outbreak is different, meaning valacyclovir can potentially take less or more time than the figures listed above to provide relief and control over a shingles infection or herpes outbreak.

Our guide to how long valacyclovir (Valtrex) takes to become effective covers more about valacyclovir’s onset of action and average time to start working for different viral injections.

Valacyclovir Brand Names, Dosages and Interactions

Valacyclovir is available as a generic medication or under several brand names. In the United States, valacyclovir is most commonly sold as Valtrex, which is the original trade name for the drug marketed by GlaxoSmithKline.

Today, valacyclovir is available as a generic medication, meaning there are numerous different trade names in use. Most of the companies manufacturing valacyclovir offer it in several doses, including 500mg and 1,000mg tablets.

The recommended dosage of valacyclovir varies based on the type of HSV or VZV virus that’s being treated, as well as your age. Generally, the most common dosages are as follows:

  • For adults with shingles, 1,000mg of valacyclovir three times daily, for a total of seven days. It’s recommended to begin treatment within 72 hours of noticing shingles.

  • For adults with cold sores, a dose of 2,000mg of valacyclovir is typically used, with a secondary dose of 2,000mg within 12 hours.

  • For adults treating a first outbreak of genital herpes, 1,000mg of valacyclovir two times daily for a total of 10 days. It’s recommended to begin treatment within 48 hours of the herpes symptoms becoming noticeable.

  • For adults with recurring genital herpes, 500mg of valacyclovir two times daily for a total of three days. It’s recommended to begin valacyclovir treatment as soon as a recurrent herpes symptom becomes visible.

It’s important to note that these are typical dosages and may not be appropriate for you based on your symptoms, age, body weight and general health. The best approach for treating any virus, including HSV or VZV, is talk to your doctor and follow their treatment advice.

It’s especially important to talk to your doctor if you have a weakened immune system or kidney disease, as these health conditions can affect the safety of valacyclovir treatment. Breastfeeding or pregnant women should also discuss the risks of valacyclovir treatment with their doctor.

Valacyclovir is known to potentially interact with other antiviral and immunosuppressant drugs, particularly those used in HIV/AIDS management. Medications that can potentially interact with valacyclovir include foscarnet, tenofovir, mycophenolate, zidovudine and the varicella virus and zoster virus vaccines.

If you use any of these medications, you should discuss potential interactions with your doctor before considering valacyclovir or any other herpes treatment.

Valacyclovir Side Effects

Valacyclovir has several potential side effects, the most common of which (affecting more than one percent of users) are vomiting, diarrhea, headache and nausea.

Less common side effects (affecting less than one percent of users) include the following:

  • Vertigo

  • Dizziness

  • Confusion

  • Sore throat

  • Rash

  • Renal impairment

  • Constipation

  • Abdominal pain

  • Agitation

  • Edema

  • Weakness

Very uncommon side effects (affecting less than 0.1 percent of users) include the following:

  • Seizures

  • Neutropenia

  • Leukopenia

  • Fatigue

  • Anorexia

  • Anaphylaxis

  • Stevens-Johnson syndrome

  • Hepatitis

  • Toxic epidermal necrolysis

  • Psychotic symptoms

Learn More About Valacyclovir (Valtrex)

Valacyclovir is one of the most common herpes medications, meaning it’s very likely to be your first choice if you’re dealing with an HSV-1 (cold sores) or HSV-2 infection (genital herpes). Our guides to valacyclovir cover every aspect of the medication, including:

This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment. Learn more about our editorial standards here.