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How Fast Does Valacyclovir Work for Cold Sores?

Kelly Brown MD, MBA

Reviewed by Kelly Brown MD, MBA

Written by Geoffrey C. Whittaker

Published 05/15/2018

Updated 07/16/2024

If you feel a cold sore coming on, antiviral drugs like valacyclovir can help manage blisters, support your immune system, and reduce those cold sore symptoms. But how fast does valacyclovir work for cold sores?

Valacyclovir, often sold under the brand name Valtrex®, is approved by the Food and Drug Administration for the treatment of cold sores (known as herpes labialis), genital herpes, and other viral infections such as herpes zoster (shingles), and chicken pox.

For most of these conditions, valacyclovir starts working as soon as you take it and reduces how long a cold sore lasts by one day. That said, it all depends on how quickly you start treatment after your first symptoms and what dose of valacyclovir you use.

And if you’re taking valacyclovir for genital herpes, the math is totally different.

Read on to find out more.

For most people, cold sores usually go away on their own, but it can take seven to 10 days for them to heal completely. And while cold sores popping up repeatedly can be annoying, you might notice they clear up faster the second time around.

If you take valacyclovir, it will usually start treating symptoms as soon as it’s in your system. However, it can take several days to see any noticeable improvements.

Valacyclovir doesn’t cure the herpes virus, and the medication might not stop a cold sore altogether. But it can speed up the healing process.

To learn more, check out our guide on how valacyclovir works.

How Long Does Valacyclovir Take to Work For Oral Herpes?

How long it takes valacyclovir to work for cold sores and fever blisters may depend on when you start taking it. Unfortunately, valacyclovir may be less effective if you start taking it after the spot or ulcer appears or after the symptoms of a cold sore develop, but we don’t know for sure.

Clinical trials show that when participants with recurrent cold sores took valacyclovir early — most of them within two hours of the onset of symptoms — their cold sores lasted an average of one day less compared to those taking a placebo.

You should start taking valacyclovir as soon as you notice signs of a cold sore, such as:

  • Tingling

  • Itching

  • Burning

Valacyclovir dosage can also make a difference in how quickly it works.

One study gave participants with cold sores one of two treatments or a placebo. The first treatment was 2 grams of valacyclovir taken twice in one day. Compared to the placebo, this reduced the duration of cold sores by one day.

The second treatment was 2 grams of valacyclovir twice in one day and 1 gram of valacyclovir twice the next day. This also reduced the duration of the cold sores, but not by as much. Compared to the placebo, it cut the duration of the cold sores by half a day.

If you miss a dose, valacyclovir may take longer to work. If you forget a pill, take the missed dose as soon as you remember, and take your next dose at its regular time. If it’s almost time to take your next dose by the time you remember, skip the missed dose.

How Long Does Valtrex Take to Work For Genital Herpes?

Valacyclovir can reduce how long initial herpes outbreaks last by two to four days. An initial herpes outbreak is your first outbreak of herpes.

Cold sores and genital herpes are close cousins. Cold sores are usually caused by herpes simplex virus type 1 (HSV-1), and genital herpes is typically caused by herpes simplex virus type 2 (HSV-2). You can learn more in our guide to HSV-1 and HSV-2.

As with cold sores, you want to start treating genital herpes as soon as they pop up. Valacyclovir is most effective when taken within 48 hours of the onset of symptoms. It’s unclear how effective valacyclovir will be if you take it 72 hours after symptoms start.

If you’ve had genital herpes before, you need to move even faster — it’s unclear whether valacyclovir will work if you start taking it 24 hours after symptoms begin for recurrent outbreaks. But we know that taking Valtrex after a cold sore appears means treatment will often take longer.

The lesson here? Start treatment as soon as you notice symptoms. No time to mess around.

Clinical trials done on people with genital herpes for the first time found that when people took a 10-day treatment of valacyclovir within 72 hours of symptoms:

  • The median time for the pain to stop was five days.

  • The median time for lesions to heal was nine days.

For people with recurrent genital herpes who started treatment within 24 hours, research shows:

  • The median time for the pain to stop is three days.

  • The median time for lesions to heal is four days.

One study found that a three-day valacyclovir treatment helped lesions from recurrent genital herpes heal in about four days. The study also looked at a one-day treatment of famciclovir, which had similar results.

You can learn about other antiviral medications in our guide to valacyclovir versus acyclovir versus famciclovir.

Rx

Keep breakouts at bay

HSV-1 can be passed on in your saliva, so you may infect someone if they drink from your water bottle or if you kiss, for example. It’s unclear whether valacyclovir can prevent cold sores from being passed on.

Genital herpes can be passed on by having sex or by touching a lesion.

Valtrex can lower the transmission rate of genital herpes. It’s not clear when exactly valacyclovir kicks in to prevent transmission, but it doesn’t take long.

In the clinical trials on people with genital herpes we mentioned earlier, a five-day treatment of valacyclovir helped reduce the average time of viral shedding. This is when your body is producing infectious particles and you can pass herpes on to someone else.

The clinical trials found that:

  • In people with first-time genital herpes, viral shedding was reduced to three days.

  • In people with recurrent genital herpes, viral shedding was reduced to two days.

You can pass on herpes even when you don’t have visible symptoms, so you may need to take valacyclovir daily to protect your partner from catching the virus.

A 2004 study looked at almost 1,500 couples where one person in each couple had genital herpes and the other was herpes-free.

The person with the virus took either 500 milligrams of valacyclovir or a placebo once a day for eight months.

At the end of the experiment, 1.9 percent of the herpes-free people with a valacyclovir-taking partner caught the virus. Among the herpes-free people with a partner who took the placebo, 3.6 percent caught the virus.

The study concluded that once-a-day valacyclovir treatment “significantly reduces the risk of transmission.”

One thing to be aware of is that this study only looked at monogamous, heterosexual couples. It’s unclear how effective valacyclovir is for homosexual couples or those with multiple partners.

FYI: You should still use a condom to help reduce the transmission of herpes, even if you’re taking valacyclovir. Seek medical advice if you’re worried about passing herpes onto your partner.

Valacyclovir converts into acyclovir, an antiviral drug, in your body. Acyclovir has a half-life of 2.5 to 3.3 hours in people with normal kidney function.

Half-life is the amount of time it takes for a drug to reduce by half in your system. So, if you took 1 gram of valacyclovir, you’d have 0.5 grams in your system 2.5 to 3.3 hours later.

However, the FDA also states that there can still be acyclovir in your urine and feces four days after taking a 1-gram dose of valacyclovir.

There’s no cure for the herpes virus, but that doesn’t mean you have to take valacyclovir for the rest of your life. If you’re treating a cold sore outbreak, you may only need to take valacyclovir for one day.

For genital herpes, daily antiviral meds like valacyclovir can reduce how often herpes outbreaks happen and lower the risk of passing on the virus to other people.

Daily treatment is often recommended for folks who get very painful or regular flare-ups or those concerned about spreading the herpes infection. This is known as suppressive therapy.

According to the FDA, it’s not known whether valacyclovir suppressive therapy is effective and safe beyond the one-year mark for people with normal immune function or beyond the six-month mark in people with HIV.

The 2004 study of couples we mentioned earlier only lasted eight months, so more studies need to be done.

Rx

Rx strength treatment

Valacyclovir should get to work as soon as you take it, and you may notice a difference in a few days. That said, how fast it actually works depends on when you take it, how much you take, and what you’re taking it for (cold sores or genital herpes).

Here’s what we do know:

  • Valacyclovir can speed up healing. When you take valacyclovir, your cold sores may last one day less than if you didn’t take any medication. It may not sound like much, but when you’ve got a cold sore, you want to return to normal as quickly as possible.

  • You need to act fast to get results. Take valacyclovir as soon as you notice the first signs of symptoms. Look out for tingling, itching, or burning.

  • For genital herpes, valacyclovir can stop the pain in five days. It can also help lesions heal up in nine days.

Timing is everything when it comes to how quickly valacyclovir can work. The drug isn’t available over the counter, though. You’ll need to speak to a healthcare provider to get a prescription.

You can chat with a licensed healthcare professional and get valacyclovir online as part of our sexual health services.

10 Sources

  1. Abudalu, M., et. al. (2008). Single-day, patient-initiated famciclovir therapy versus 3-day valacyclovir regimen for recurrent genital herpes: a randomized, double-blind, comparative trial. https://academic.oup.com/cid/article/47/5/651/296163.
  2. American College of Obstetricians and Gynecologists. (n.d.). Genital Herpes. https://www.acog.org/womens-health/faqs/genital-herpes.
  3. Centers for Disease Control (n.d.). Genital Herpes – CDC Detailed Fact Sheet. https://www.cdc.gov/std/herpes/stdfact-herpes-detailed.htm.
  4. Corey, L. et al. (2004). Once-daily valacyclovir to reduce the risk of transmission of genital herpes. https://www.nejm.org/doi/full/10.1056/NEJMoa035144.
  5. National Library of Medicine. (2018). What are the treatment options for genital herpes? https://www.ncbi.nlm.nih.gov/books/NBK525771/.
  6. Saleh, D., et. al. (2023). Herpes Simplex Type 1. https://www.ncbi.nlm.nih.gov/books/NBK482197/.
  7. Spruance, S. L. et al. (2003). High-dose, short-duration, early valacyclovir therapy for episodic treatment of cold sores: results of two randomized, placebo-controlled, multicenter studies. https://journals.asm.org/doi/10.1128/aac.47.3.1072-1080.2003.
  8. Valtrex (Valacyclovir Hydrochloride) Caplets. (n.d.). https://www.accessdata.fda.gov/drugsatfda_docs/label/2008/020487s014lbl.pdf.
  9. World Health Organization. (n.d.). Herpes simplex virus. https://www.who.int/news-room/fact-sheets/detail/herpes-simplex-virus.
  10. Worrall G. (2009). Herpes labialis. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2907798/.
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, MBA

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 was previously Medical Director of a male fertility startup where she lead 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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