6 Valacyclovir (Valtrex) Side Effects You Should Know About

Valacyclovir (Valtrex) is an antiviral medication that’s used to treat a variety of conditions, including herpes, shingles, and chickenpox. Some people who are prescribed valacyclovir only take it for one day. But others have to take it for longer periods of time — even months or years.

You can manage common valacyclovir side effects, like headaches and nausea, easily at home. However, more serious side effects that require medical attention are also possible. Below, we cover what you should know about six valacyclovir side effects and how to manage them.

1. Headache

Valacyclovir can cause headaches, especially if you’re taking it every day to prevent herpes outbreaks. But keep in mind that your headaches could be caused by a variety of things. So, when you have a headache, take note of where you feel the pain and how long it’s been going on. Your healthcare provider can then help you pinpoint the cause.

There are several tips and tricks you can try to help prevent or relieve headaches, whether they’re a side of valacyclovir or caused by something else. If needed, talk to your healthcare provider about taking an over-the-counter (OTC) pain reliever, like acetaminophen (Tylenol), for relief. 

Let your provider know if your headaches worsen or if they don’t go away after trying OTC treatments.

2. Nausea

Nausea is a common side effect of many medications, including valacyclovir. You can take valacyclovir with or without food. But taking it with a meal or snack may lessen the likelihood of nausea. If that doesn’t help, or if you start vomiting after taking valacyclovir, contact your healthcare provider. They may have other recommendations to help you manage this side effect. 

3. Abdominal pain

Some people taking valacyclovir report having abdominal pain. Let your healthcare provider know if you experience severe or worsening abdominal pain after taking the medication. Like headaches, there are many potential causes of abdominal pain, so your provider can help you determine if your symptoms are related to valacyclovir.

Abdominal pain accompanied by other symptoms — such as bleeding, fever, or shortness of breath — is a sign of something more serious. If you experience this combination of symptoms, you may need to go to your nearest emergency room.

4. Kidney problems

In rare cases, valacyclovir can cause kidney problems, including acute kidney failure. Risk factors that increase the likeliness of this side effect include older age, taking higher valacyclovir doses with existing kidney problems, and dehydration. Combining valacyclovir with other medications that are hard on the kidneys, like ibuprofen (Advil, Motrin), can also increase this risk.

There are a few things you can do to minimize your risk of having kidney problems related to valacyclovir. These include staying hydrated and taking the medication as prescribed. Also, make sure your healthcare provider and pharmacist have your current medication list to check for potential interactions. 

Decreased urination, swelling in your legs or ankles, and/or chest pain can be symptoms of acute kidney failure. So let your healthcare provider know right away if you experience any of these symptoms.

5. Agitation, confusion, and hallucinations

Valacyclovir can cause side effects that affect your brain. The most common examples include agitation, confusion, and hallucinations. 

Experts don’t really know why these side effects occur. But they can happen to anyone taking valacyclovir. It appears to be more likely in older adults and people with kidney problems, though.

While taking valacyclovir, watch for any changes in your behavior or thoughts. And call your healthcare provider or go to the closest ER if you notice anything unusual. 

6. Blood problems

Thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are very serious valacyclovir side effects that affect your blood. But they’re also quite rare.

With these syndromes, you develop small blood clots that can then affect organs throughout your body. TPP also causes fever and other brain-related effects, such as headache and confusion. Your provider may order bloodwork to confirm the cause of your symptoms.

During clinical trials, certain people with weakened immune systems developed TTP and HUS. This included people with advanced HIV and those who were taking higher valacyclovir doses after certain types of organ transplants.

Early symptoms of TTP and HUS can include nausea, vomiting, and bloody diarrhea. And abdominal pain and fever are also possible. As the syndromes progress, they can cause shortness of breath and changes in urination. So contact your healthcare provider right away if you experience these symptoms.

When should you contact your healthcare provider about valacyclovir side effects?

The most common valacyclovir side effects are usually mild and can be managed at home. And, if you’re only taking valacyclovir for a few days, they should subside after you complete treatment. But if you have side effects that are getting worse or not going away, contact your healthcare provider. 

While rare, serious valacyclovir side effects are also possible. Symptoms like bloody diarrhea, changes in your thoughts and behaviors, and shortness of breath can be indicators of a more serious problem that requires immediate medical attention.

If your side effects feel severe or life-threatening, consider going to the nearest ER.

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Written by:

Brian Leonard, PharmD, BCACP, BCGP

Brian Leonard earned his doctorate in pharmacy from the University of Florida College of Pharmacy and is board certified in Ambulatory Care and Geriatric Pharmacy.

Edited by:

Kaitlyn Ellis, PharmD Candidate

She has 5 years of experience in various areas of the medical field; including women's health, community pharmacy, and hospital pharmacy experience.

Reviewed by:

Austin Ulrich, PharmD, BCACP

Austin Ulrich, PharmD, BCACP, is a board-certified ambulatory care clinical pharmacist. In his early career, he provided direct patient care in a variety of settings, including hospital and community pharmacies, and in a primary care clinic as a clinical pharmacist.

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