Valtrex (valacyclovir) is generally well tolerated, and most people who take it experience only mild side effects or none at all. The most common issues are headache, nausea, and abdominal pain. Serious side effects are rare but can affect the kidneys and nervous system, particularly in older adults and people with reduced kidney function.
Common Side Effects
The side effects most people notice while taking Valtrex are gastrointestinal. Nausea, stomach pain, and occasionally vomiting are the top complaints. Headache is also frequently reported. These tend to be mild and often improve as your body adjusts to the medication.
In pediatric studies of children ages 1 to 12, diarrhea and dehydration were the most frequently noted side effects, each occurring in about 7% of participants. No children in those trials experienced serious adverse events or needed to stop treatment.
How Valtrex Works in Your Body
Valtrex is a prodrug, meaning your body converts it into acyclovir after you swallow it. That conversion happens rapidly in the intestines and liver. The advantage of taking Valtrex instead of acyclovir directly is better absorption: Valtrex delivers roughly three to five times more of the active drug into your bloodstream, which means fewer pills per day. Once converted, acyclovir interferes with the virus’s ability to copy its DNA, slowing or stopping an outbreak.
This higher absorption is also why side effects can sometimes be more noticeable than with plain acyclovir tablets, especially at higher doses.
Kidney-Related Risks
The most clinically significant concern with Valtrex is its effect on the kidneys. Acyclovir can form crystals in the kidney’s filtering tubes, and when that happens it can trigger acute kidney injury. This risk is highest in people who are dehydrated, elderly, already have kidney disease, or are taking other medications that stress the kidneys.
If you have reduced kidney function, dosing needs to be adjusted. The FDA label specifies different doses based on how well your kidneys filter, measured by creatinine clearance. For example, someone with moderately reduced kidney function taking Valtrex for shingles would take half the standard frequency, while someone with severely impaired kidneys might take a quarter of the usual dose. Your prescriber will typically check your kidney function before starting treatment if there’s any reason for concern.
Staying well hydrated while taking Valtrex is one of the simplest things you can do to reduce this risk. Drinking plenty of water helps prevent crystal formation in the kidneys.
Nervous System Side Effects
Some people, particularly older adults and those with chronic kidney problems, can develop neurological symptoms while on Valtrex. These typically show up as confusion, hallucinations, dizziness, irritability, tremor, or difficulty with coordination. In rare cases, seizures can occur.
These symptoms usually appear within the first three days of starting treatment and resolve within about five days after stopping the medication. The connection to kidney function matters here too: when the kidneys can’t clear acyclovir efficiently, drug levels build up in the blood and eventually affect the brain. This is why dose adjustments for kidney impairment are so important.
Rare but Serious Complications
At very high doses (8 grams per day, far above what most people are prescribed), Valtrex has been linked to a dangerous blood clotting disorder called TTP/HUS. This condition, which affects red blood cells and platelets, has occurred in patients with advanced HIV and in bone marrow or kidney transplant recipients. Some of those cases were fatal. At the standard doses used for cold sores, genital herpes, and shingles, this complication has not been a significant concern for otherwise healthy people.
Long-Term Use and Safety
Many people take Valtrex daily for months or years as suppressive therapy for recurrent genital herpes. The CDC notes that long-term safety and efficacy have been well documented for daily valacyclovir use. Allergic and other adverse reactions are rare, and neither routine lab monitoring nor periodic treatment breaks are considered necessary. Antiviral resistance from long-term use is also uncommon.
This is reassuring if you’re on suppressive therapy or considering it. The side effect profile doesn’t appear to worsen over time, and there’s no established pattern of liver enzyme changes or other lab abnormalities emerging with prolonged use.
Safety During Pregnancy and Breastfeeding
Both acyclovir and valacyclovir have good safety profiles in pregnant women. Registry data and clinical trials have not found an association between valacyclovir use and congenital malformations or infant toxicity, whether exposure happens in the womb or through breast milk.
Acyclovir does pass into breast milk. In one study, it was detectable in 80% of breast milk samples, with a median concentration of about 2.6 micrograms per milliliter. Despite this, no adverse effects were identified in nursing infants. Many providers prescribe Valtrex during late pregnancy specifically to prevent herpes outbreaks around delivery.
Who Needs to Be Most Careful
The people most likely to experience significant side effects from Valtrex fall into a few overlapping groups:
- Older adults: Higher risk of both kidney injury and neurological symptoms, even with normal-appearing kidney function, because kidney filtration naturally declines with age.
- People with kidney disease: Reduced clearance of the drug means higher blood levels and greater risk of crystal formation in the kidneys and toxicity to the nervous system.
- People taking other kidney-stressing medications: Combining Valtrex with other drugs that affect the kidneys can compound the risk of acute kidney injury.
- Immunocompromised individuals: Transplant recipients and people with advanced HIV face the highest risk of the rare blood disorder TTP/HUS, particularly at high doses.
For most healthy adults taking standard doses for cold sores, genital herpes, or shingles, Valtrex is a well-tolerated medication. The key precautions are straightforward: drink enough water, be aware that confusion or unusual neurological symptoms in the first few days warrant prompt attention, and make sure your prescriber knows about any kidney issues before you start.