Cold sores are painful and oozing, and they always seem to appear before that wedding or class reunion. Also called fever blisters, cold sores are small, fluid-filled lesions. They typically form near or on your lips and can cause symptoms such as tingling, itching, or burning.

There’s no cure for cold sores. However, because they’re caused by a virus, they can be treated with antiviral medications. These include the prescription medication Valtrex. Valtrex, which contains the active ingredient valacyclovir, can help your cold sores clear up faster. It can also reduce the number of cold sores you get. Read on to learn how Valtrex works and how to use it to treat your cold sores.

Cold sores typically heal on their own within about four to six days. (The first cold sore you get will likely last longer.) Therefore, most people don’t require treatment for their cold sores. In some cases, though, a doctor may prescribe an antiviral medication such as Valtrex. They may do this if you get cold sores often, for example. They may also do it if you’re at high risk of serious complications, such as from a weakened immune system.

To treat a cold sore, you take Valtrex on the day you notice a cold sore forming. Valtrex works by preventing the herpes virus from growing and spreading.

Your doctor may also prescribe Valtrex to help prevent future cold sores, which is an off-label use. In that case, you and your doctor would work together to create the best treatment plan for you.

Learn more: Cold sore causes, symptoms, and treatments »

Valtrex is an oral caplet. It comes in 500-milligram and 1-gram strengths. It’s available as a brand-name product as well as a generic medication (valacyclovir).

For adults and children 12 years and older

The recommended dosage is 2 grams twice daily, taken 12 hours apart, for one day. Valtrex should be started at the earliest signs of a cold sore.

For children 11 years and younger

Valtrex is not recommended for treating cold sores in children of this age group. (However, it can be used to treat chickenpox in children ages 2 years and older.)

In clinical studies, Valtrex worked best for people who took it within two hours of their first cold sore symptoms. In general, these people had shorter cold sore episodes by about one day compared to people who didn’t take Valtrex at all. However, there was no marked difference between people who didn’t take the drug and those who took Valtrex after their cold sores had already formed.

  • Take Valtrex at the first sign of a cold sore.
  • You can take it with or without food.
  • Don’t take more than the prescribed number of caplets each day.
  • If your child can’t swallow caplets, ask your pharmacist to make the caplets into an oral suspension (liquid).
  • Be sure to drink lots of water. It’s important to stay well hydrated while you take Valtrex.

The more common side effects of Valtrex include:

  • headache
  • dizziness
  • nausea
  • vomiting
  • stomach pain

The serious side effects of Valtrex can include:

Serious side effectSymptoms
kidney failure in people at risk of kidney damagefever, back pain in area where kidneys are located, tiredness, trouble making urine
nervous system problems and unusual mood or behaviordepression, aggression, unsteady movements, confusion, speech problems*, hallucinations, seizures, coma
low blood cell countstiredness, increased infections
allergic reactionrash, swollen mouth and throat, breathing problems
* such as slurred speech and not making sense while speaking

Valtrex may not be the best choice for certain people.

People with kidney damage or kidney failure may need a lower dosage of Valtrex. Be sure to tell your doctor if you have kidney problems before you start taking the drug.

If you’ve ever had an allergic or other serious reaction to Valtrex, Zovirax (acyclovir), or the ingredients in them, do not take Valtrex without talking to your doctor first.

For more information about Valtrex, talk with your doctor. Feel free to review this article with them and ask any questions you have, such as:

  • Is it important that I take medication to prevent cold sores?
  • Are there drug-free ways to help avoid cold sores?
  • Are there over-the-counter drug options that I could consider?

Together, you and your doctor can decide if Valtrex or another medication or treatment is a good choice to treat your cold sores. For more information, read about the six best cold sore remedies.

Q:

Are cold sores contagious?

A:

Yes. They spread from person-to-person contact, such as kissing. You can pass cold sores to another person even when the sores are not visible.

Healthline Medical TeamAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.