Valacyclovir (Valtrex) is a prescription antiviral used for infections caused by herpes viruses, including cold sores, genital herpes, and shingles. It does not eliminate herpes from the body, but it can reduce symptoms, shorten outbreaks, and in some cases reduce transmission risk when used as prescribed.
Common Side Effects
Common effects include headache, nausea, and stomach discomfort. Some people may also experience dizziness or other mild symptoms. Contact a healthcare professional if side effects are severe or persistent.
Kidney and Nervous-System Risks
Valacyclovir is removed mainly through the kidneys. People with kidney impairment, older adults, or those who become significantly dehydrated may have a higher risk of drug accumulation and serious side effects.
Seek urgent medical care for confusion, hallucinations, severe agitation, seizures, markedly reduced urination, or severe weakness. Kidney-related dosing may need adjustment by the prescriber.
Serious Skin Reactions
The FDA updated valacyclovir labeling in 2025 to include severe cutaneous adverse reactions. Get urgent medical help for a widespread rash, blistering or peeling skin, sores in the mouth or eyes, or a rash with fever or facial swelling.
Children, Pregnancy, and Breastfeeding
Valacyclovir is approved for certain pediatric uses, so it is incorrect to say that all children under 12 must avoid it. Age, condition, weight, and formulation matter.
If you are pregnant, planning pregnancy, or breastfeeding, tell your clinician. Valacyclovir may be used in pregnancy when medically appropriate, but the decision should be individualized.
Drug Interactions
Do not rely on generic online interaction lists. Tell your doctor or pharmacist about all prescription medicines, over-the-counter medicines, supplements, and herbal products. Kidney-toxic medicines and medicines that affect renal function may be especially relevant.
How to Use It Safely
- Take valacyclovir exactly as prescribed.
- Do not double a missed dose.
- Maintain normal hydration unless your healthcare professional has told you to restrict fluids.
- Do not stop or change treatment early without medical advice.
Bottom Line
Valacyclovir is widely used and often well tolerated, but kidney problems, neurologic symptoms, and serious skin reactions need prompt attention. Individual dosing matters, especially in older adults and people with kidney disease.
Reliable Sources
Last reviewed for medical accuracy: September 2026.