If you have certain viral infections due to the herpes simplex virus, you may be interested in learning more about valacyclovir. It’s a generic prescription drug used to treat cold sores in adults and some children. It can also be used to treat shingles or to treat or prevent genital herpes in adults. This drug is also approved to treat chickenpox in some children.

Valacyclovir comes as tablet that you swallow. In most cases, it’s used as a short-term treatment option. But in some cases, your doctor may recommend that you take the drug long term.

This article describes valacyclovir’s side effects (also called adverse effects). For more information about valacyclovir, including details about its uses, see this in-depth article.

Some people may experience mild to serious side effects during valacyclovir treatment. Examples of the drug’s commonly reported side effects include:

* To learn more about this side effect, see the “Side effects explained” section below.

You may experience mild side effects from valacyclovir. Examples of mild side effects that have been reported with this drug include:

* To learn more about this side effect, see the “Side effects explained” section below.
† An allergic reaction is possible after taking valacyclovir. Although allergic reactions weren’t reported in studies, they were reported after the drug was approved for use.

These side effects are usually temporary. And some may be easily managed. But if you have any symptoms that are ongoing or bother you, talk with your doctor or pharmacist. And do not stop taking valacyclovir unless your doctor recommends it.

Valacyclovir may cause mild side effects other than those listed above. See the drug’s prescribing information for details.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with valacyclovir, visit MedWatch.

Although less common, it’s also possible to experience serious side effects from taking valacyclovir. Serious side effects that have been reported with this drug include:

If you develop serious side effects while taking valacyclovir, call a doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

* To learn more about this side effect, see the “Side effects explained” section below.
† An allergic reaction is possible after taking valacyclovir. Although allergic reactions weren’t reported in studies, they were reported after the drug was approved for use.

Children taking valacyclovir experienced many of the same side effects reported in adults taking the drug. But children also experienced:

If your child or a child you care for is taking valacyclovir and you have questions or concerns about side effects of the drug, talk with a doctor or pharmacist.

Below are answers to some frequently asked questions about valacyclovir’s side effects.

Are there any long-term side effects of valacyclovir?

Yes, valacyclovir can cause long-term side effects, though these were not common in the drug’s studies. For example, valacyclovir may cause kidney problems or changes to your thinking or behavior, such as confusion or hallucinations. These side effects may continue even if you stop treatment with the drug.

If you have questions or concerns about valacyclovir’s long-term side effects, talk with a doctor or pharmacist.

Is hair loss a side effect of this drug?

It’s possible you could experience hair loss from taking valacyclovir, though this wasn’t a side effect reported in studies of the drug. It’s been reported as a possible side effect since the Food and Drug Administration (FDA) approved the drug. But it’s not known for sure if valacyclovir caused the hair loss.

If you notice hair loss during valacyclovir treatment, talk with a doctor or pharmacist. They can help determine the cause and the best way to treat it.

Does valacyclovir affect your liver?

It’s possible for valacyclovir to affect your liver, though this wasn’t reported in studies of the drug.

Liver problems, including increased liver function tests and hepatitis, were reported after valacyclovir was approved by the FDA. But it’s not known for sure if valacyclovir was the cause.

It’s important to be aware of symptoms of liver problems, so you can tell your doctor right away if you experience any. Symptoms may include:

If you experience liver problems while taking valacyclovir, your doctor can help to determine the cause and treat the condition.

Learn more about some of the side effects valacyclovir may cause.

Kidney problems

Although rare, it’s possible for valacyclovir to cause kidney problems. Specifically, this drug may increase the risk of acute kidney failure in:

What might help

Your doctor may recommend monitoring you for symptoms of kidney problems throughout your treatment with valacyclovir. Symptoms may include:

You should drink plenty of water while taking valacyclovir to help prevent dehydration. Staying hydrated may reduce your risk of developing acute kidney failure as a side effect.

If you develop symptoms of kidney problems, your doctor will likely have you stop taking valacyclovir. They can recommend the best treatment for your kidney problems as well as the condition you’re taking valacyclovir to treat.

Headache

It’s possible for valacyclovir to cause headache. This was one of the most common side effects reported by those taking the drug.

What might help

Talk with your doctor if you experience headaches from valacyclovir. If they’re severe or bothersome, your doctor may be able to recommend ways to decrease this side effect. For example, they may recommend taking an over-the-counter pain medication, such as ibuprofen (Advil) or acetaminophen (Tylenol).

Nausea

You may experience nausea from taking valacyclovir. This was one of the most common side effects reported by people taking the drug.

What might help

If you feel nauseous during your valacyclovir treatment, talk with a doctor or pharmacist. They may be able to recommend ways to decrease this side effect. For example, taking medications with food can sometimes help.

Allergic reaction

Like most drugs, valacyclovir can cause an allergic reaction in some people. While this side effect wasn’t reported in studies, it has been seen since the drug came on the market. Symptoms can be mild to serious and can include:

  • skin rash
  • itchiness
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin, usually in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call a doctor right away. They may suggest a treatment to manage your symptoms. Examples include:

  • an antihistamine you take by mouth, such as Benadryl (diphenhydramine)
  • a product you apply to your skin, such as hydrocortisone cream

If the doctor confirms you’ve had a mild allergic reaction to valacyclovir, they’ll decide if you should continue taking it.

If you have symptoms of a severe allergic reaction, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If the doctor confirms you’ve had a serious allergic reaction to valacyclovir, they may have you switch to a different treatment.

Keeping track of side effects

During your valacyclovir treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you start taking new drugs or using a combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how it affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Keeping notes and sharing them with your doctor will help them learn more about how valacyclovir affects you. They can then use this information to adjust your treatment plan if needed.

Valacyclovir may not be right for you if you have certain medical conditions. (This is known as a drug-condition interaction.) Other factors may also affect whether valacyclovir is a good treatment option for you. Talk with your doctor about your health history before starting this drug. Factors to consider include those described below.

Bone marrow transplant or kidney transplant. Tell your doctor if you’ve ever had a bone marrow transplant or kidney transplant. If you have, taking valacyclovir may increase your risk of the blood conditions thrombocytopenic purpura and hemolytic uremic syndrome. Your doctor may monitor you for symptoms of these conditions, such as fever or red spots on the skin, while you’re taking valacyclovir. If you notice symptoms of these conditions, call your doctor right away.

Kidney problems, including dialysis. Tell your doctor if you have any kidney problems or if you’re on dialysis. Your kidneys remove valacyclovir from your body. So if they aren’t working well enough, the drug may build up in your system. This can increase your risk of side effects and could worsen your kidney problems. If you have kidney problems, your doctor may recommend a lower dose of valacyclovir for you.

Allergic reaction. If you’ve had an allergic reaction to valacyclovir or any of its ingredients, your doctor will likely not prescribe it for you. Ask them about other medications that might be better options.

HIV or AIDS. If you have HIV or AIDS, tell your doctor before starting valacyclovir. This is because you may have an increased risk of the blood conditions thrombocytopenic purpura and hemolytic uremic syndrome. Your doctor may monitor you for symptoms of these conditions, such as fever or red spots on the skin, throughout your valacyclovir treatment. If you notice symptoms of these conditions, call your doctor right away.

Alcohol and valacyclovir

There aren’t any known interactions between valacyclovir and alcohol. But it’s possible for alcohol to worsen valacyclovir’s side effects. For example, both alcohol and valacyclovir can cause:

  • nausea
  • vomiting
  • headache
  • dizziness

So drinking alcohol while taking valacyclovir may increase your risk of these side effects. Talk with your doctor about how much alcohol, if any, is safe for you to drink while taking this drug.

Pregnancy and breastfeeding while taking valacyclovir

It may be safe to take valacyclovir during pregnancy. Studies of pregnant women taking the drug have not shown an increased risk of problems with fetal development (commonly known as birth defects). But there’s not enough information to determine if the drug may increase the risk of miscarriage.

It’s important to note that untreated herpes simplex virus infections may cause problems to a developing fetus, such as growth problems or stillbirth.

It’s not known if valacyclovir shows up in breast milk. But valacyclovir breaks down into a drug called acyclovir that does show up in breast milk. So it’s likely that a baby who is breastfeeding will be exposed to the drug. At this time, it’s unknown what effects this drug may have on a breastfeeding baby.

If you’re pregnant, planning to become pregnant, or breastfeeding, talk with your doctor before starting valacyclovir treatment.

Side effects from valacyclovir are usually mild. But it’s also possible to experience serious side effects from this drug. If you have questions about side effects the drug can cause, talk with your doctor. You can also ask them about Valtrex, which is the brand-name version of valacyclovir. (A generic drug and its brand-name version are expected to have the same side effects because they contain the same active ingredient.*)

Here are some questions to help get your conversation started:

  • Is my risk of side effects from valacyclovir higher due to my other medical conditions?
  • How do side effects of valacyclovir compare with other drugs used to treat herpes simplex virus?
  • Does my risk of side effects from valacyclovir depend on my dosage?
  • If I experience side effects, can my dose of valacyclovir be decreased?

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*An active ingredient is what makes a drug work.

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.