Common and serious side effects of valacyclovir

Mike Bohl, MD, MPH, ALM - Contributor Avatar

Written by Chimene Richa, MD 

Mike Bohl, MD, MPH, ALM - Contributor Avatar

Written by Chimene Richa, MD 

last updated: Jul 16, 2020

2 min read

Valacyclovir (brand name Valtrex) is an antiviral medicine that works against viral infections, especially herpes infections.

The types of herpes valacyclovir treats include genital herpes, cold sores, chickenpox, and shingles. While valacyclovir is generally well-tolerated, certain groups are at higher risk of more serious side effects. Read on for more about this treatment, its side effects, and who should avoid it.

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How does valacyclovir work?

Valacyclovir treats viral infections by preventing the virus from multiplying and spreading throughout the body (Ormrod, 2000). Only available by prescription, valacyclovir is most effective if taken as soon as possible after developing the virus. Unfortunately, valacyclovir does not cure the infection. 

Taking valacyclovir can make your herpes symptoms less painful and resolve faster than they would without medication, but it cannot cure you of the virus—which can live and hide in your body. In some cases, valacyclovir is used long-term (suppressive therapy) to prevent or suppress future herpes virus outbreaks.

Valacyclovir Important Safety Information: Read more about serious warnings and safety info.

Does valacyclovir have any side effects?

While valacyclovir is generally well tolerated, some people may experience side effects. The most common side effects of valacyclovir include (UpToDate, n.d.):

  • Headache

  • Nausea

  • Abdominal pain

  • Tiredness (fatigue)

  • Depression

  • Skin rash

Certain groups are at higher risk of more serious side effects, including (UpToDate, n.d.):

  • Elderly individuals: May experience central nervous system side effects like agitation, hallucinations, confusion, seizures, etc. 

  • People with kidney disease or kidney problems: Since the kidneys remove valacyclovir from the body, people with decreased kidney function need a reduced dose. Otherwise, too much of the drug can accumulate, leading to kidney failure and symptoms like drowsiness, reduced urine output, and swelling of legs or feet (FDA, 2008). 

  • People with low immune system function: Those with low immune system function, such as people with HIV/AIDS or bone marrow transplants, may develop a medical condition that affects their blood cells like thrombotic thrombocytopenic purpura (TTP) or hemolytic uremic syndrome (HUS). 

  • Pregnant or breastfeeding women: There are no adequate and well-controlled studies of valacyclovir use in pregnant women. It should be used in pregnancy only if the potential benefit justifies the potential risk to the fetus (FDA, 2008). Valacyclovir is considered Pregnancy Category B. Breastfeeding women should use caution and seek medical advice before taking valacyclovir (FDA, 2008).

Seek medical advice at the first sign of any serious side effects. Anyone with a history of an allergic reaction to valacyclovir or acyclovir should avoid taking it.

Besides the side effects listed, valacyclovir can also interact with other prescription drugs that you may be taking. Cladribine, a chemotherapy medicine used to treat leukemia, loses its effectiveness if taken with valacyclovir. If you take foscarnet (also an antiviral drug) with valacyclovir, it increases the risk of damage to the kidney (UpToDate, n.d.). Avoid taking valacyclovir if you are also taking any of these medications to prevent drug interactions.

Taking valacyclovir at the same time as getting varicella or zoster vaccines may also interfere with the vaccine's effectiveness (UpToDate, n.d.). Avoid taking valacyclovir 24 hours before and 14 days after getting the vaccines. Always tell your healthcare provider about any medical problems you may have or drugs that you are taking before starting valacyclovir.


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

July 16, 2020

Written by

Chimene Richa, MD

Fact checked by

Mike Bohl, MD, MPH, ALM

About the medical reviewer

Dr. Mike is a licensed physician and a former Director, Medical Content & Education at Ro.