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How Fast Does Valacyclovir Work for Cold Sores?

Katelyn Hagerty FNP

Reviewed by Katelyn Hagerty, FNP

Written by Vanessa Gibbs

Published 05/15/2018

Updated 10/21/2023

You wake up one morning and feel that telltale tingling of a cold sore coming on. You want it gone — fast!

Luckily, antiviral drugs like valacyclovir (often sold under the brand name of Valtrex®) can help. But how fast does valacyclovir work for cold sores? 

In general, 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 noticing symptoms and what dose of valacyclovir you use.

Plus, if you’re taking valacyclovir for genital herpes, it’s a whole other ball game.

Read on to find out more.

Valacyclovir usually starts treating symptoms as soon as it’s in your system. But it can take several days to see any noticeable improvements. 

What’s this drug, exactly? Valacyclovir is FDA-approved for the treatment of cold sores (known as herpes labialis), genital herpes and other viral infections like herpes zoster (shingles) and chicken pox.

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 on HSV-1 and HSV-2.

Depending on which type of virus you have, you might get sores and blisters on your lips, genitals or anus — and some lucky people get no sores or symptoms at all. 

For most people, cold sores usually go away on their own. It can take seven to 10 days for a cold sore to completely heal. And as annoying as it is for cold sores to repeatedly pop up, you might notice they clear up faster the second time around. 

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

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

For Oral Herpes 

How long it takes valacyclovir to work for cold sores may depend on when you start taking it. It’s unclear how effective valacyclovir is if you start taking it after the spot or ulcer appears or after the symptoms of a cold sore develop.

You should start taking valacyclovir as soon as you notice signs of a cold sore.

These include:

  • Tingling

  • Itching

  • Burning  

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 than those taking a placebo. 

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 twice a day for one day. This reduced how long cold sores lasted by one day compared to the placebo. 

The second treatment was 2 grams of valacyclovir twice daily for one day, then 1 gram of valacyclovir twice daily for another day. This also reduced how long the cold sores lasted, but not by as much. It cut cold sore duration by half a day compared to the placebo.

If you miss a dose, valacyclovir may take longer to work. If you’ve forgotten a pill — it happens to the best of us — 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, skip the missed dose.

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. 

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 not clear how effective valacyclovir will be if you start taking it 72 hours after symptoms start.

If you’ve had genital herpes before, you need to move even faster. Why? It’s unclear whether valacyclovir will work if you start taking it 24 hours after symptoms start. 

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

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

  • The median time for 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 treatment of valacyclovir 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.

It’s unclear whether valacyclovir can prevent cold sores from being passed on. 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.

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

Valtrex can lower the transmission rate of genital herpes — good news for whoever you’re sleeping with!

It’s not clear exactly 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 on herpes 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. One person in each couple had genital herpes (not something you’d put on your Bumble profile), 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 placebo-taking partner, 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.

And 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 renal (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 of it in your system 2.5 to 3.3 hours later.

The FDA (U.S. Food and Drug Administration) 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 and 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.

Valacyclovir should get to work as soon as you take it, and you may notice a difference in a few days. That said, it really 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. It can make cold sores last one day less compared to not taking any medication. It may not sound like much, but when you’ve got a cold sore, you want to get back to normal — STAT

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

  • For genital herpes, valacyclovir can make the pain stop in five days. And lesions can heal up in nine days. 

Timing is everything when it comes to how quickly valacyclovir can work for you. 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. Valtrex (Valacyclovir Hydrochloride) Caplets. (n.d.).
  2. Saleh, D., Yarrarapu, S.N.S., Sharma, S. (2023, July 24). Herpes Simplex Type 1 - StatPearls. NCBI.
  3. Herpes simplex virus. (n.d.).
  4. Genital Herpes. (n.d.).
  5. Worrall G. (2009). Herpes labialis. BMJ clinical evidence, 2009, 1704.
  6. 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. Antimicrobial agents and chemotherapy, 47(3), 1072–1080.
  7. What are the treatment options for genital herpes? (2018, July).
  8. Abudalu, M., Tyring, S., Koltun, W., Bodsworth, N., & Hamed, K. (2008). Single-day, patient-initiated famciclovir therapy versus 3-day valacyclovir regimen for recurrent genital herpes: a randomized, double-blind, comparative trial. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America, 47(5), 651–658.
  9. Genital Herpes – CDC Detailed Fact Sheet. (n.d.).
  10. Corey, L. et al. (2004). Once-daily valacyclovir to reduce the risk of transmission of genital herpes. The New England journal of medicine, 350(1), 11–20.
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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.

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