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Valacyclovir: How It Works, Side Effects & More

Kelly Brown MD, MBA

Reviewed by Kelly Brown, MD

Written by Rachel Sacks

Published 05/15/2018

Updated 03/03/2024

Having safe sex to prevent sexually transmitted infections (STIs) may seem like a no-brainer. But we also understand the very real stigma that comes with an STI like herpes.

Herpes is caused by two viruses: the herpes simplex virus type 1 (HSV-1 or cold sores) and the herpes simplex virus type 2 (HSV-2 or genital herpes). This STI is actually very common, with over 500,000 new infections in 2018 in the U.S. alone.

The bad news is that there’s no cure for herpes. The good news is that consistent treatment can manage the severity of herpes outbreaks.

Plus, various treatment options are available, including Valtrex® — or valacyclovir, the generic version — an antiviral medication.

So what is valacyclovir? Can this medication be used to stop a cold sore? Are there adverse effects, and if so, how long do Valtrex side effects last?

Dealing with herpes and starting a new medication can be overwhelming. We’re here to answer the above questions and go over other key information about valacyclovir.

Is valacyclovir an antibiotic or another type of medication? And is there a difference between the Valtrex- the brand-name medication, and valacyclovir- the generic name medication?

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

As mentioned above, valacyclovir is Valtrex’s generic name — this medication is available under a few different brand names.

But you don’t have to worry about generic versus brand-name medication. Generic drugs are required by the FDA (U.S. Food and Drug Administration) to be the same dosage, safety, effectiveness, strength and quality as their brand-name counterparts.

Valacyclovir is an extremely well-studied medication prescribed to people of all ages. It’s been widely and successfully used to control and treat the symptoms of HSV-1 and HSV-2. This medication is so effective, in fact, it’s listed as the “gold standard” in herpes treatment studies.

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Now that you know what this medication is, you might be wondering: How does valacyclovir work?

We mentioned that valacyclovir is an antiviral drug, meaning the medication fights infections caused by viruses. With that said, valacyclovir isn’t classified as an antibiotic — antibiotics are medications that help the immune system fight infections caused by bacteria, not viruses.

Like many medications, valacyclovir works through a complex chemical process that interferes with how the herpes virus reproduces. In other words, it stops the herpes virus from spreading to healthy cells, reducing the effects of the virus.

The drug slows 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). It converts into acyclovir (another antiviral herpes medication) after it passes through the liver.

Comparing valacyclovir versus acyclovir, valacyclovir has a far higher level of bioavailability. This means more of the drug will make it into your body than other herpes medications that are broken down by the liver.

Valacyclovir helps sores that developed from the virus heal faster while making the aches, cold symptoms and other signs of herpes less severe.

Oh, and if you’re curious about having sex with herpes, you’ll be glad to know valacyclovir  can lower the transmission rate of the virus — meaning it reduces its ability to spread.

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 those with HSV-2 who didn’t use medication.

But know that valacyclovir isn’t a 100 percent effective solution for preventing herpes. Herpes infections are contagious, and you can infect other people even while taking the medication.

How fast valacyclovir works depends on a number of factors, from the type of infection you have to how quickly you start treatment after noticing symptoms. But on average, most people see an improvement in symptoms within 10 days.

Valacyclovir dosages also vary. Several dosage forms are available, each based on the type of HSV or VZV virus being treated and the patient’s age.

Dosages include:

  • A 1,000-milligram dose of valacyclovir, given three times daily for seven days, is typically prescribed for shingles. It’s recommended to begin treatment within 72 hours of noticing shingles.

  • A 2,000-milligram dose of valacyclovir is used for adults with cold sores, with a secondary dose of 2,000 milligrams within 12 hours.

  • If treating a first outbreak of genital herpes, adults take 1,000 milligrams of valacyclovir two times daily for 10 days, beginning treatment within 48 hours of the first sign of herpes symptoms.

  • Adults with recurring genital herpes are usually prescribed 500 milligrams of valacyclovir two times daily for three days. It’s recommended to begin valacyclovir treatment as soon as a recurrent herpes symptom becomes visible.

While these are typical dosages, your prescription will be as unique as you and tailored to your symptoms, age and general health.

If you miss a dose, take it as soon as you remember. Or skip the missed dose if it’s almost time to take your next dose — don’t double the dose to catch up.

The best treatment plan for herpes? Seeking medical advice from a healthcare provider and following their recommendations.

It’s especially crucial to talk to a healthcare provider if you have kidney disease or a weakened immune system, as these medical conditions can affect the safety of valacyclovir treatment.

Breastfeeding or pregnant women should also discuss the risks of valacyclovir treatment with their healthcare provider, as the medication can pass into breast milk.

Like any medication, there are possible side effects of valacyclovir. The most common valacyclovir side effects are usually vomiting, diarrhea, headache and nausea.

There’s also a chance of experiencing less common side effects of valacyclovir:

  • Vertigo

  • Dizziness

  • Confusion

  • Sore throat

  • Rash

  • Renal impairment

  • Constipation

  • Abdominal pain

  • Agitation

  • Edema

  • Weakness

Even more uncommon side effects include:

  • Seizures

  • Leukopenia (low white blood cell count)

  • Neutropenia (low levels of a certain white blood cell)

  • Fatigue

  • Anorexia

  • Severe allergic reaction

  • Stevens-Johnson syndrome (a rare skin disorder)

  • Hepatitis

  • Psychotic symptoms

Get in touch with your provider if you experience any of these adverse effects.

Valacyclovir Interactions

Valacyclovir is known to potentially interact with other antiviral and immunosuppressant drugs, particularly those used in HIV/AIDS management.

Medications that might interact with valacyclovir include foscarnet, tenofovir, mycophenolate and zidovudine, as well as the varicella virus and zoster virus vaccines.

Again, the best way forward is to talk to a healthcare professional about potential drug interactions if you use any prescription drugs or over-the-counter supplements before you take valacyclovir or any other herpes treatment.

Herpes and cold sores are an unfortunate possibility if you’re sexually active. But there are ways to manage symptoms. Valacyclovir is among the leading treatment options.

  • What is valacyclovir? Despite its hard-to-pronounce name, valacyclovir is neither a comic book villain nor a dinosaur species. It’s an antiviral drug used to treat herpes simplex virus (HSV) and varicella-zoster virus (VZV, more commonly known as chickenpox) infections. One common brand name for valacyclovir is Valtrex®.

  • How does valacyclovir work? The drug slows down the growth of herpes, making it easier for the body to control the infection, and reduces symptoms. Although there isn’t a cure for herpes, valacyclovir can help manage symptoms like aches and sores.

  • What are the side effects? There’s a possibility of experiencing mild side effects, most commonly vomiting, diarrhea, headache or nausea. Valacyclovir can also interact with other medications, including other antiviral and immunosuppressant drugs.

If you’re dealing with herpes (or think you might have the virus) and are interested in valacyclovir as a treatment, talk to your healthcare provider about your symptoms. They can come up with a treatment plan for your unique needs.

You can also connect with a healthcare professional to learn about what sexual health treatments are available.

10 Sources

  1. STD Facts - Genital Herpes. (2022, January 3). CDC. Retrieved from
  2. VALTREX- valacyclovir hydrochloride tablet, film coated. (n.d.). DailyMed. Retrieved from
  3. Generic Drug Facts. (2021, November 1). FDA. Retrieved from
  4. Jiang, Y. C., Feng, H., Lin, Y. C., & Guo, X. R. (2016). New strategies against drug resistance to herpes simplex virus. International journal of oral science, 8(1), 1–6. Retrieved from
  5. What You Should Know about Flu Antiviral Drugs. (n.d.). CDC. Retrieved from
  6. Sauerbrei A. (2016). Herpes Genitalis: Diagnosis, Treatment and Prevention. Geburtshilfe und Frauenheilkunde, 76(12), 1310–1317. Retrieved from
  7. Bonnar P. E. (2009). Suppressive valacyclovir therapy to reduce genital herpes transmission: good public health policy?. McGill journal of medicine : MJM : an international forum for the advancement of medical sciences by students, 12(1), 39–46. Retrieved from
  8. HIGHLIGHTS OF PRESCRIBING INFORMATION These highlights do not include all the information needed to use VALTREX safely and effec. (n.d.). Retrieved from
  9. Valtrex (Valacyclovir Hydrochloride) Caplets. (n.d.). Retrieved from
  10. Sykes JE, Papich MG. (2014). Antiviral and Immunomodulatory Drugs. Canine and Feline Infectious Diseases. Retrieved from
Editorial Standards

Hims & Hers has strict sourcing guidelines to ensure our content is accurate and current. We rely on peer-reviewed studies, academic research institutions, and medical associations. We strive to use primary sources and refrain from using tertiary references. See a mistake? Let us know at [email protected]!

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.

Kelly Brown MD, MBA
Kelly Brown, MD

Dr. Kelly Brown is a board certified Urologist and fellowship trained in Andrology. She is an accomplished men’s health expert with a robust background in healthcare innovation, clinical medicine, and academic research. Dr. Brown is a founding member of Posterity Health where she is Medical Director and leads strategy and design of their Digital Health Platform, an innovative education and telehealth model for delivering expert male fertility care.

She completed her undergraduate studies at University of North Carolina at Chapel Hill (go Heels!) with a Bachelor of Science in Radiologic Science and a Minor in Chemistry. She took a position at University of California Los Angeles as a radiologic technologist in the department of Interventional Cardiology, further solidifying her passion for medicine. She also pursued the unique opportunity to lead departmental design and operational development at the Ronald Reagan UCLA Medical Center, sparking her passion for the business of healthcare.

Dr. Brown then went on to obtain her doctorate in medicine from the prestigious Northwestern University - Feinberg School of Medicine and Masters in Business Administration from Northwestern University - Kellogg School of Management, with a concentration in Healthcare Management. During her surgical residency in Urology at University of California San Francisco, she utilized her research year to focus on innovations in telemedicine and then served as chief resident with significant contributions to clinical quality improvement. Dr. Brown then completed her Andrology Fellowship at Medical College of Wisconsin, furthering her expertise in male fertility, microsurgery, and sexual function.

Her dedication to caring for patients with compassion, understanding, as well as a unique ability to make guys instantly comfortable discussing anything from sex to sperm makes her a renowned clinician. In addition, her passion for innovation in healthcare combined with her business acumen makes her a formidable leader in the field of men’s health.

Dr. Brown is an avid adventurer; summiting Mount Kilimanjaro in Tanzania (twice!) and hiking the incredible Torres del Paine Trek in Patagonia, Chile. She deeply appreciates new challenges and diverse cultures on her travels. She lives in Denver with her husband, two children, and beloved Bernese Mountain Dog. You can find Dr. Brown on LinkedIn for more information.

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