Valacyclovir is a generic drug that’s prescribed for certain viral infections in adults and certain children. As with other drugs, valacyclovir can cause side effects, such as headache and nausea.

Valacyclovir is available as the brand-name drug Valtrex.

The active ingredient in valacyclovir is valacyclovir hydrochloride. It comes as an oral tablet.

Read on to learn about possible common, mild, and serious side effects of valacyclovir. For a general overview of the drug, including details about its uses, see this article.

Valacyclovir can cause side effects (also referred to as adverse effects). Some of these are more common than others. These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

These are just a few of the more common side effects reported by people who took valacyclovir in clinical trials:

  • abdominal pain
  • runny nose and scratchy throat
  • headache*
  • nausea*
  • dizziness*

* For more information about this side effect, see “Valacyclovir: Side effect specifics” below.

Mild side effects can occur with valacyclovir. This list doesn’t include all possible mild side effects of the drug. For more information, you can refer to valacyclovir’s prescribing information.

Mild side effects that have been reported with valacyclovir include:

These side effects may be temporary, lasting a few days to weeks. However, if the side effects last longer than that, bother you, or become severe, be sure to talk with your doctor or pharmacist.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you develop a side effect while taking valacyclovir and want to tell the FDA about it, visit MedWatch.

* For more information about this side effect, see “Valacyclovir: Side effect specifics” below.

Valacyclovir may cause serious side effects. The list below may not include all possible serious side effects of the drug. For more information, you can refer to valacyclovir’s prescribing information.

If you develop serious side effects while taking valacyclovir, call your 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.

Serious side effects that have been reported and their symptoms include:

* For more information about this side effect, see “Valacyclovir: Side effect specifics” below.
† An allergic reaction is possible after taking valacyclovir. However, this side effect wasn’t reported in clinical trials.

Clinical trials showed that children who took valacyclovir experienced the following side effects:

These side effects are mild and typically go away on their own. If your child has side effects that are severe, bothersome, or won’t go away, talk with their doctor.

* HSV is the virus that causes certain herpes infections, such as cold sores.

Valacyclovir may cause several side effects. Here are some frequently asked questions about the drug’s side effects and their answers.

Does valacyclovir have any long-term side effects?

Yes, valacyclovir can cause long-term side effects. This includes side effects that may occur after taking the drug for a long time. This also includes side effects that can lead to long-term complications.

Examples of valacyclovir’s long-term side effects include:

In clinical trials of valacyclovir, the highest risk of long-term side effects occurred in people who took the drug for suppressive therapy. With this therapy, you take a lower dose of valacyclovir for a long time. The goal is to suppress (reduce) the risk of the viral infection returning.

Your doctor can work with you to help manage these and other side effects you have with valacyclovir. To learn more, talk with them.

* To learn more, see “Valacyclovir: Side effect specifics” below.
† This side effect wasn’t seen in clinical trials, but it has been reported since valacyclovir was approved and put on the market.

Is hair loss a side effect of valacyclovir?

Hair loss was not reported as a side effect of valacyclovir in clinical trials, but it may occur. Since the drug was approved and put on the market, there have been rare reports of hair loss. The exact reason why this side effect may occur is not fully known.

If you’re concerned about hair loss and valacyclovir, talk with your doctor.

Should I expect to develop itching while taking valacyclovir?

No, itching is not an expected side effect of valacyclovir.

Itching is a common symptom of herpes simplex virus (HSV), which valacyclovir may be used to treat. Types of HSV infections include cold sores and genital herpes.

When you start taking valacyclovir, you may still experience symptoms of infection (such as itching). This is because it takes time for valacyclovir to work. People often confuse symptoms of infection with side effects of valacyclovir.

If the itching becomes bothersome while taking valacyclovir, talk with your doctor. They may suggest treatments to help ease it.

Does valacyclovir cause liver-related side effects?

Liver-related side effects were not reported in clinical trials of valacyclovir. However, there have been rare reports of a liver problem called hepatotoxicity since the drug was approved and put on the market. With hepatotoxicity, your liver becomes damaged and inflamed due to medications or harmful substances.

Symptoms of liver damage or inflammation can include:

  • abdominal pain
  • itchiness
  • nausea or vomiting
  • jaundice

If you have any of these side effects while taking valacyclovir, be sure to talk with your doctor. They may order blood tests to check for elevated levels of liver enzymes, which could be a sign of liver damage. If you have hepatotoxicity, your doctor will likely have you switch to a different medication.

Do 500-mg and 1-g valacyclovir oral tablets cause the same side effects?

Yes, both the 500-milligram (mg) and 1-gram (g) oral tablets of valacyclovir can cause the same side effects. However, clinical trials showed that people who took 1 g of valacyclovir reported certain side effects more often than people who took 500 mg of valacyclovir. These side effects include pain and cramping with menstruation, abdominal pain, and dizziness.*

Keep in mind that taking higher strengths and dosages of valacyclovir will lead to an increased level of the drug in your body. And having more of the drug in your system could increase your risk of side effects.

Your doctor and pharmacist can help answer additional questions you have about valacyclovir, its strengths, and its side effects.

* To learn more, see “Valacyclovir: Side effect specifics” below.

Learn more about some of the side effects that valacyclovir may cause. To find out how often side effects occurred in clinical trials, see the prescribing information for valacyclovir.

Headache

Headache is a common side effect that may occur while taking valacyclovir. Clinical trials showed that headaches were often mild to moderate.

Symptoms of headaches can include:

  • dull pain in your head
  • pain on both sides of the head
  • pain in the front and back of your head

What you can do

If you start to experience headaches that happen often or are severe while you’re taking valacyclovir, you should talk with your doctor. They may recommend over-the-counter medications such as ibuprofen (Motrin, Advil, others) or acetaminophen (Tylenol) to help manage the pain.

Nausea

Nausea is a common side effect of valacyclovir. With nausea, you may feel sick and as if you need to vomit. Clinical trials showed that nausea was typically mild and did not lead to people stopping valacyclovir treatment.

What you can do

Mild nausea will typically go away on its own. If your nausea symptoms bother you or don’t ease, talk with your doctor. They can help figure out whether valacyclovir or another factor is the cause.

To help relieve your nausea, your doctor may recommend lifestyle changes, such as staying hydrated. Depending on the severity of your symptoms, they may also prescribe medication for nausea. This could include ondansetron (Zofran) or metoclopramide (Reglan).

Dizziness

Dizziness can occur with valacyclovir. With dizziness, you may feel lightheaded and unable to maintain your balance. In clinical trials of people who took valacyclovir, dizziness was rare and mild.

What you can do

Mild dizziness typically goes away on its own. However, you should talk with your doctor if your dizziness symptoms are bothersome. They help determine whether valacyclovir is the cause. Your doctor may recommend lifestyle changes or medication as needed.

Central nervous system toxicity

A serious central nervous system (CNS) problem can occur with valacyclovir. It’s called CNS toxicity and may also be referred to as neurotoxicity. With this condition, your CNS (brain and spinal cord) becomes damaged and starts to work differently after being exposed to something harmful.

Clinical trials of valacyclovir showed that CNS toxicity was rare. The condition was more likely to occur at higher doses and in people with a kidney problem, such as chronic kidney disease.

Symptoms of CNS toxicity during valacyclovir treatment can include:

What you can do

If you experience any symptoms of CNS toxicity, talk with your doctor right away. There are many different causes of CNS toxicity. Your doctor may recommend that you stop taking valacyclovir so the cause can be determined.

Allergic reaction

As with most drugs, valacyclovir can cause an allergic reaction in some people. However, this side effect wasn’t reported in clinical trials.

Symptoms can be mild or serious and can include:

  • skin rash or itching
  • flushing
  • difficulty breathing
  • swelling under your skin, typically in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe
  • chest pain
  • fainting

What you can do

For mild symptoms of an allergic reaction, call your doctor right away. They may recommend ways to ease your symptoms and determine whether you should keep taking valacyclovir. However, if your symptoms are serious and you think you’re having a medical emergency, immediately call 911 or your local emergency phone number.

Be sure to talk with your doctor about your health history before you take valacyclovir. This drug may not be the right treatment for you if you have certain medical conditions or other factors that affect your health. These are considered drug-condition or drug-factor interactions. The conditions and factors to consider include:

HIV. If you have HIV and take valacyclovir, you may have an increased risk of developing certain conditions that can cause blood clots. These are thrombotic thrombocytopenia purpura and hemolytic uremic syndrome.

Your doctor may monitor your blood cell counts while you take valacyclovir. If you start to develop symptoms of these conditions, they may recommend a different medication. (For symptoms, see “Valacyclovir: Serious side effects” above.)

Allergic reaction. If you’ve had an allergic reaction to valacyclovir or any of its ingredients, your doctor will likely not prescribe the drug. Taking valacyclovir could cause you to have another allergic reaction. Ask your doctor what other medications may be better options for you.

Kidney disease. Valacyclovir can cause kidney damage. If you have a history of chronic kidney disease or kidney damage, taking the drug could increase your risk of acute (sudden) kidney failure.

Before starting valacyclovir treatment, tell your doctor about any kidney problems you have. They’ll assess how well your kidneys are working. In some cases, they may prescribe a lower dose of valacyclovir or recommend a different medication.

Pregnancy and breastfeeding while taking valacyclovir

Here’s some information about pregnancy, breastfeeding, and valacyclovir.

Pregnancy. It may be safe to take valacyclovir during pregnancy, but more information is needed. According to clinical trials and animal studies, valacyclovir does not cause congenital anomalies (commonly known as birth defects) when taken during pregnancy.

Your doctor may prescribe valacyclovir if you’re pregnant and have or have had genital herpes. This is because the infection can pass to your child during pregnancy or childbirth.

If you’re pregnant or planning a pregnancy, talk with your doctor. They can help determine whether valacyclovir is right for you.

Breastfeeding. Valacyclovir passes into breast milk. However, it’s not known what effect this may have on a child who is breastfed.

If you’re breastfeeding or planning to do so, talk with your doctor before taking valacyclovir. They can talk with you about the risks and benefits of breastfeeding while taking this drug. Your doctor can also recommend other ways to feed your child.

Side effects of valacyclovir are typically mild to moderate in severity and go away on their own. If you experience symptoms of serious side effects while taking valacyclovir, talk with your doctor right away.

Valacyclovir is typically taken short term. It may sometimes be prescribed for longer periods of time, depending on the type of viral infection.

If you’d like to learn more about valacyclovir, talk with your doctor or pharmacist. They can help answer any questions you have about side effects from taking the drug.

Referring to the following articles about valacyclovir can provide you with additional information:

  • More information about valacyclovir. For details about other aspects of valacyclovir, refer to this article.
  • Cost. If you’d like to learn about valacyclovir and cost, see this article.

Disclaimer: Medical News Today 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.