Key takeaways:
Valacyclovir (Valtrex) is an oral, prescription-only medication. It’s used to treat certain types of viral infections, including shingles and herpes.
Common valacyclovir side effects include headaches, nausea, and abdominal pain. While rare, kidney problems, brain-related side effects, and blood disorders are also possible.
A few ways to lower the risk of valacyclovir side effects are taking your doses with food and staying hydrated. Your healthcare team should also check for potential drug interactions that can make experiencing side effects more likely.
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Valacyclovir (Valtrex) is an antiviral medication that’s used to treat different viral infections, including herpes, shingles, and chickenpox. Some people who are prescribed valacyclovir only take it for 1 day. But others have to take it for longer periods of time — even months or years.
No matter how long you’re taking valacyclovir or brand-name Valtrex, it’s a good idea to understand how it may affect your body. Like with all medications, side effects are possible. And knowing what to expect can help you recognize potential issues early and have informed conversations with your prescriber.
Here’s what you should know.
Valacyclovir side effects at a glance
Many common valacyclovir (Valtrex) side effects can be managed at home. But more serious side effects may require medical attention. The type and severity of side effects you experience can depend on factors such as your age, your overall health, and how long you take valacyclovir.
Common valacyclovir side effects reported during clinical trials include:
Headaches
Nausea
Abdominal pain
Menstrual cramps
Depression
Joint pain
Vomiting
Cold and flu-like symptoms
Dizziness
Diarrhea
Fever
Less common but potentially serious valacyclovir side effects include:
Lab test changes
Serious blood clotting disorders in certain people
Kidney problems
Brain-related side effects, such as agitation, hallucinations, and confusion
Below, we dig deeper into six valacyclovir side effects and how to manage them.
1. Headaches
Valacyclovir can cause headaches, especially if you’re taking it every day to prevent herpes outbreaks. But keep in mind that headaches can occur for many reasons. It’s a good idea to keep a headache journal, taking note of potential triggers to help pinpoint the cause.
In the meantime, there are steps you can take to prevent or relieve headaches:
Stay hydrated. Drink plenty of water throughout the day, since dehydration can cause or worsen headaches.
Limit alcohol and caffeine. These substances can sometimes make headaches worse.
Try an over-the-counter (OTC) pain reliever. If appropriate, you could take acetaminophen (Tylenol) for relief. It’s best to avoid pain relievers that are hard on the kidneys, such as ibuprofen (Advil, Motrin).
Stick to a consistent sleep schedule. Poor sleep quality and lack of sleep can trigger headaches.
Finding relief: Dealing with a painful, itchy shingles rash? Learn about over-the-counter, prescription, and home-remedy options for relief.
Pharmacist tips: Experts cover how long valacyclovir (Valtrex) takes to work for shingles and other tips for taking it.
Typical dosage: Here’s what you should know about typical valacyclovir dosages and what to do if you miss your dose.
Tell your prescriber if your headaches become more frequent or severe, or don’t improve with OTC treatments.
2. Nausea
Nausea is a common side effect of many medications, including valacyclovir. While you can take valacyclovir with or without food, taking it with a meal or snack may help reduce stomach discomfort. Start by pairing your doses with something bland and easy to digest, such as toast, crackers, or a banana.
If your nausea continues, try these additional tips:
Stay upright for at least 30 minutes after taking a dose to help it settle.
Avoid greasy or spicy foods, which can make nausea worse.
Sip fluids slowly throughout the day to stay hydrated, especially if you feel queasy.
If you still feel nauseated after trying these tips, or if you start vomiting, contact your prescriber.
3. Abdominal pain
Some people taking valacyclovir report having abdominal pain. This pain is often mild and can be managed at home. If you experience mild stomach discomfort after taking a dose, try taking the medication with food to see if that helps.
Since abdominal pain can occur for many reasons, it’s a good idea to keep a symptom log to help pinpoint the cause. Take note of when the pain occurs, how long it lasts, and what it feels like. Tell your prescriber if the pain:
Is severe or gets worse over time
Interferes with eating, sleeping, or daily activities
Doesn’t improve after you try taking your doses with food
Seek immediate medical care at the nearest ER if you have abdominal pain accompanied by serious symptoms, such as:
Blood in your stool or vomit
Fever or chills
Dizziness or fainting
Difficulty breathing or shortness of breath
4. Kidney problems
Although rare, valacyclovir can cause kidney problems, especially in people with certain risk factors. Examples of these risk factors include:
Older age
Preexisting kidney problems
Dehydration
Taking valacyclovir with other medications that are hard on the kidneys — such as nonsteroidal anti-inflammatory drugs (NSAIDS) — and taking higher dosages also increase the risk.
To help reduce your risk of kidney problems while taking valacyclovir:
Stay hydrated, especially if you’re taking a higher dosage or using valacyclovir long term.
Take valacyclovir exactly as prescribed.
Avoid NSAIDs like ibuprofen, unless your healthcare team says it’s safe to take them with valacyclovir.
Share your current medication list with your healthcare team so they can screen for any harmful interactions.
Decreased urination, swelling in your legs or ankles, and shortness of breath can be symptoms of acute kidney failure. Tell your healthcare team right away if you experience any of these symptoms. Go to the nearest ER if any symptoms feel severe or life-threatening.
5. Agitation, confusion, and hallucinations
Valacyclovir can sometimes affect the brain, which can lead to side effects such as agitation, confusion, or hallucinations. This risk for these side effects appears to be higher for older adults and people with kidney problems.
To help catch problems early:
Pay attention to any unusual changes in mood, behavior, or thinking.
Let someone close to you know that you’re starting valacyclovir, so they can help spot changes if you don’t notice them yourself.
If you or someone else notices new or worsening mental changes, call your prescriber right away. If your symptoms are severe or you feel unsafe, go to the nearest ER for evaluation.
6. Blood disorders
Thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) are very serious valacyclovir side effects that affect the blood. But they’re also quite rare.
These syndromes cause small blood clots that can affect organs throughout the body. TPP also causes fever and other brain-related effects, such as headaches and confusion. If you experience these symptoms, your healthcare team may order blood work to confirm the cause.
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 dosages after certain types of organ transplants.
Early symptoms of TTP and HUS can include nausea, vomiting, and bloody diarrhea. Abdominal pain and fever are also possible. As the syndromes progress, they can cause shortness of breath and changes in urination. Contact your prescriber right away if you experience these symptoms.
Frequently asked questions
People who’ve had an allergic reaction to valacyclovir or acyclovir in the past shouldn’t take Valtrex. Special precautions or dosage adjustments are needed for older adults and those with kidney disease, since these groups are at higher risk for certain side effects.
While taking Valtrex, it’s best to avoid:
Becoming dehydrated: Drink plenty of fluids to help protect your kidneys.
Taking medications that are hard on the kidneys: The risk of kidney damage increases when these medications are combined with Valtrex.
Performing tasks that require focus in some cases: If you experience side effects such as dizziness or confusion, it’s best to avoid driving or performing other tasks that require focus until you feel better.
Yes, it’s possible to have herpes outbreaks while taking Valtrex. But taking Valtrex consistently as prescribed can help reduce how often outbreaks happen and make symptoms less severe when they do occur.
People who’ve had an allergic reaction to valacyclovir or acyclovir in the past shouldn’t take Valtrex. Special precautions or dosage adjustments are needed for older adults and those with kidney disease, since these groups are at higher risk for certain side effects.
While taking Valtrex, it’s best to avoid:
Becoming dehydrated: Drink plenty of fluids to help protect your kidneys.
Taking medications that are hard on the kidneys: The risk of kidney damage increases when these medications are combined with Valtrex.
Performing tasks that require focus in some cases: If you experience side effects such as dizziness or confusion, it’s best to avoid driving or performing other tasks that require focus until you feel better.
Yes, it’s possible to have herpes outbreaks while taking Valtrex. But taking Valtrex consistently as prescribed can help reduce how often outbreaks happen and make symptoms less severe when they do occur.
The bottom line
Common valacyclovir (Valtrex) side effects include headaches, nausea, and abdominal pain. These side effects are usually mild and can be treated at home. But serious side effects — such as kidney problems, brain-related effects, and blood disorders — are also possible and require immediate medical care.
Talk to your prescriber or pharmacist if you have any questions about taking valacyclovir.
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References
Aurobindo Pharma. (2024). Valacyclovir hydrochloride tablet, film coated [package insert]. DailyMed.
Bhandari, J., et al. (2023). Hemolytic uremic syndrome. StatPearls.
Brandariz-Nuñez, D., et al. (2021). Neurotoxicity associated with acyclovir and valacyclovir: A systematic review of cases. Journal of Clinical Pharmacy and Therapeutics.
MedlinePlus. (2024). Acute kidney failure.
Nagarakanti, S. R., et al. (2024). Is there a role for chronic suppressive therapy in herpes simplex virus infection? Cleveland Clinic Journal of Medicine.
Stanley, M., et al. (2023). Thrombotic thrombocytopenic purpura. StatPearls.
Trachtman, H. (2013). HUS and TTP. Pediatric Clinics of North America.
