Valtrex for Cold Sores: Everything You Need to Know

By Zina Semenovskaya, MD
Medically reviewed checkmarkMedically reviewed
July 21, 2022

Valtrex (Valacyclovir hydrochloride) is an antiviral medication used to treat viral infections like cold sores. Cold sores are caused by the highly contagious herpes simplex virus (HSV-1). 

Cold sores are also called fever blisters and appear as small clustered lumps around the lip or mouth. They’re usually itchy, tingly, painful, and filled with fluid. 

If you have cold sores, Valtrex is one of the treatment options for you. 

In this article, I’ll discuss what cold sores are and how Valtrex works to treat and prevent them. I’ll explain the dosage for Valtrex and the side effects of taking it.

I’ll talk about the risks and interactions that come with taking Valtrex. Finally, I’ll explore Valtrex’s effectiveness for cold sores. 

What Are Cold Sores?

Cold sores (herpes labialis) are small, clustered lumps that appear around the mouth.

They’re caused by the herpes simplex virus (HSV).

There are two strains of HSV: type 1 (HSV-1)and type 2 (HSV-2). 

Generally, type 1 HSV causes oral herpes, or cold sores, while type 2 causes genital herpes, though each type can cause both.

Herpes is spread through close physical contact like kissing, sexual contact, or sharing an infected person’s spoons or towels. 

It’s possible to be infected with HSV-1 and not show any symptoms, but most people develop cold sores around their lips or inside their mouths at least once.

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How Does Valtrex Work for Cold Sores?

Antiviral medications don’t kill viruses directly, so cold sores can’t be cured—they can only be treated to manage the symptoms.

Antivirals stop viruses from causing more harm by suppressing their ability to multiply or develop. 

Valtrex (valacyclovir) is a prescription-only antiviral medication recommended for treating cold sores and preventing outbreaks in people who get cold sores often.

Valtrex is a prodrug of acyclovir, meaning it turns into acyclovir once it’s in your body. 

Valtrex works on cold sores by binding to the DNA of the herpes virus and stopping it from multiplying, preventing outbreaks, and shortening the duration of symptoms once they develop. 

Dosage of Valtrex 

The recommended dose of Valtrex for the treatment of a cold sore outbreak for adults is 2g every 12 hours for one day only.

It’s best to start treatment for cold sores as soon as you notice the symptoms of an outbreak coming on (itching, lumps, and burning).

Valtrex starts working not long after it’s been taken, but it can take a couple of days for the symptoms to clear up. 

Side Effects

Like all medications, valacyclovir may cause some side effects, although not everyone experiences them.

Some side effects are more common than others.

Common Side Effects

Common side effects of Valtrex include:

These common side effects are usually no cause for alarm, and improve within a day or two. 

Less Common Side Effects

Some uncommon side effects of Valtrex include: 

  • Confusion
  • Hallucination (seeing or hearing things that aren’t real)
  • Drowsiness
  • Tremors
  • Hive-like rash
  • Blood in urine
  • Lower back pain
  • Reduced number of white blood cells (leucopenia)

In rare cases, people may experience: 

  • Difficulty speaking or slurred speech
  • Delirium
  • Seizures
  • Kidney problems
  • Trouble with brain functions
  • Lack of coordination (shaky movements/unsteadiness when walking)
  • Coma

Some side effects are severe and require immediate medical attention.

You should stop using Valtrex if you notice:

  • Swelling of the face, lips, neck, throat, or tongue
  • Difficulty breathing
  • Itchy skin rash and flushing
  • Reduced blood pressure 

These side effects could be signs of a severe, potentially life-threatening allergic reaction called anaphylaxis. 

Risks and Interactions

Drug interactions can change how medications work and may increase your chances of experiencing serious side effects.

Following your healthcare provider’s instructions on how to take your medications will reduce your likelihood of experiencing any unpleasant side effects. 

Don’t take Valtrex if you’re allergic to valacyclovir, acyclovir, or any of the medication’s ingredients.

Also, talk to your healthcare provider before taking Valtrex if you have:

  • Kidney problems
  • Liver problems
  • A weakened immune system 

To prevent unwanted drug interactions, let your doctor know about any medications, supplements, and vitamins you’re taking.

Your healthcare provider should especially know if you’re taking any of the following medications because they have known interactions with Valtrex:

  • Probenecid (Benemid)
  • Cimetidine (Tagamet)
  • Foscarnet (Foscavir)
  • Methotrexate (Rasuvo)
  • Pentamidine (Pentam)
  • Tacrolimus (Astagraf XL)
  • Cyclosporin (Gengraf)
  • Aminoglycosides

Valacyclovir is not typically recommended for pregnant people. So if you’re pregnant, thinking about getting pregnant, or breastfeeding, talk to your healthcare provider before taking Valtrex.

They’ll weigh the risks and decide if taking the medication in your condition is worth it or not. 

Valacyclovir may cause you to feel drowsy, which can affect your ability to drive or work with delicate equipment.

Avoid driving or using heavy machinery while on Valtrex until you know how your body reacts to the medication.

Is Valtrex Effective for Cold Sores?

Valtrex is effective for treating cold sores in adults and children.

Valtrex has a long duration of action and high bioavailability, meaning it can be taken only a few times and still have strong effects against cold sores.

It starts to work not long after it’s taken. 

Not only does Valtrex clear out the symptoms of cold sores, but people who take it also have lower chances of spreading the virus and infecting others.

How to Use

Valtrex is a prescription-only medication, so you should only take it when your healthcare provider prescribes it, and take it exactly as directed. 

For cold sores, the usual dose is 2g of Valtrex twice a day.

Take the second dose 12 hours after the first, and if you’re taking Valtrex for cold sores, don’t take it for more than one day. 

While you take Valtrex, make sure to:

  • Take it by mouth
  • Swallow the tablets whole with water
  • Follow your healthcare provider’s advice for taking the medicine
  • Drink a lot of water if you’re over 65 or have kidney problems

Valtrex is not usually harmful, but if you take too much of it, you may feel sick, agitated, vomit, hallucinate, or go into a coma. 

Always take the medication as prescribed, and do not let anyone use your medication even if they have similar symptoms.

Remember to talk to your health providers if you’re unclear on any areas or have questions concerning Valtrex. 

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When to See a Medical Provider

If you notice any serious adverse effects while taking Valtrex, seek medical advice immediately. You should also see a medical provider if: 

  • You have frequent cold sore outbreaks
  • The cold sores don’t heal within ten days of taking Valtrex
  • There are sores in your mouth, or you have swollen gums
  • You have a weakened immune system 
  • You think your cold sores might be something else 
  • You have questions concerning your medication

How K Health Can Help

Get quick medical attention for all your health concerns. Did you know you can get affordable primary care with the K Health app?

Download K Health to check your symptoms, explore conditions and treatments, and if needed text with a healthcare provider in minutes. 

K Health’s AI-powered app is based on 20 years of clinical data.

Frequently Asked Questions

How quickly does Valtrex work for cold sores?
Valtrex treats cold sores fairly quickly. A simple one-day treatment with Valtrex is enough to start reducing the cold sores. However, the cold sore blisters may take a few days to clear off completely.
Is Valtrex the best for cold sores?
Yes, the best treatment for cold sores is Valtrex. It prevents the HSV-1 virus from spreading and reduces the cold sore symptoms.
Can I take Valtrex every day for cold sores?
Valtrex oral tablets are taken twice a day for just one day to treat cold sore outbreaks. But for people prone to frequent cold sore outbreaks, Valtrex can be taken daily to reduce the chances of the cold sore returning.
How much Valtrex should I take during an outbreak?
For cold sores, 2g of Valtrex should be taken twice in one day, 12 hours apart. Remember, Valtrex for cold sores is a one-day treatment, and you shouldn't take it for more than one day. For treatment of recurrent outbreaks of genital herpes, adults take 500mg of Valtrex two times a day for three days.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Zina Semenovskaya, MD

Dr. Semenovskaya specializes in emergency medicine, and received her medical degree from Weill Cornell Medical College. She is currently the medical director at Remote Emergency Medicine Consulting, LLC and splits her time working clinically as an emergency medicine attending in California and Alaska. She is the first of our doctors to be fluent in Russian.

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