Valacyclovir is generally considered safe during pregnancy. Clinical data collected over several decades have not identified an increased risk of major birth defects associated with the drug or its active form, acyclovir. It remains one of the most commonly prescribed antivirals for pregnant women with genital herpes, and guidelines in multiple countries recommend it as suppressive therapy in late pregnancy to reduce the chance of passing herpes to the baby during delivery.
What the Birth Defect Data Show
The most direct concern for any medication in pregnancy is whether it increases the risk of birth defects, particularly when taken during the first trimester when organs are forming. A large study published in JAMA found that among 229 infants exposed to valacyclovir in the first trimester, 3.1% were diagnosed with a major birth defect. In the general unexposed population, the baseline rate was 2.4%. That difference was not statistically significant, meaning it fell within the range of normal variation and could easily be due to chance rather than the drug itself.
The FDA’s current labeling for valacyclovir reflects this body of evidence, stating that clinical data in pregnant women “have not identified a drug associated risk of major birth defects.” The labeling does note that data on miscarriage risk are more limited, which is common for many medications simply because miscarriage is harder to study systematically.
How the Drug Reaches the Baby
Valacyclovir is a prodrug, meaning your body quickly converts it into acyclovir after you take it. Acyclovir does cross the placenta, but at a relatively slow rate. Studies using human placental tissue found that acyclovir transfers from the maternal to fetal side at about 30% the rate of substances that cross freely. The placenta does not break down or alter the drug, and it moves in both directions at similar rates, so it doesn’t accumulate on the fetal side. This moderate, passive transfer is one reason the safety profile has remained reassuring across decades of use.
Suppressive Therapy Before Delivery
If you have a history of genital herpes, the biggest risk to your baby comes during vaginal delivery if you’re having an active outbreak. To prevent this, clinical guidelines recommend starting suppressive valacyclovir at 36 weeks of pregnancy and continuing until delivery. The standard regimen is 500 mg taken twice daily.
This approach makes a measurable difference. In a randomized trial, only 4% of women taking valacyclovir had a recurrent herpes outbreak at delivery, compared to 13% in the placebo group. Viral shedding detected by culture dropped from 9% to 2%. Fewer outbreaks at delivery means fewer emergency cesarean sections performed solely to protect the baby from exposure. No infants in either group developed neonatal herpes, and there were no significant differences in neonatal complications.
Side Effects During Pregnancy
Pregnant women taking valacyclovir typically experience the same mild side effects as anyone else on the drug. In clinical trials, the most commonly reported issues were headache, cough, vomiting, vaginal discharge, and urinary tract infections, all of which occurred at similar rates in women taking a placebo. One trial found a slightly higher rate of fever in the valacyclovir group, but researchers considered this unlikely to be related to the medication since fever has not been a recognized side effect in the broader population.
At standard suppressive doses, serious side effects are rare. However, when valacyclovir is used at much higher doses for other conditions (such as treating cytomegalovirus infection during pregnancy), a small percentage of women, roughly 1.7% in one review, experienced temporary kidney problems that resolved after stopping the medication. This is relevant mainly for higher-dose regimens and not typical herpes suppression.
Use Beyond Herpes
Valacyclovir is increasingly being studied for preventing mother-to-child transmission of cytomegalovirus (CMV), a common virus that can cause serious problems if a baby is infected before birth. A meta-analysis of eight studies covering 618 women found that valacyclovir treatment reduced the risk of the baby becoming infected by about 63% when the mother contracted CMV during the first trimester. Treated pregnancies also had a higher likelihood of the baby being born without symptoms even if infection occurred. This is a newer and still-evolving use of the drug, typically involving higher doses than herpes suppression, but the safety profile in these studies was consistent with what’s been seen in herpes treatment.
Safety While Breastfeeding
If you’re taking valacyclovir after delivery, very little of the drug makes it into breast milk. Valacyclovir itself is undetectable in milk because it converts to acyclovir so quickly in your body. The acyclovir that does appear in milk results in an infant dose of roughly 0.6 mg per kilogram per day from a standard 500 mg twice-daily maternal dose. That’s about 1% of the dose that would be given directly to a newborn being treated for herpes with IV acyclovir. At these trace levels, no adverse effects in breastfed infants have been reported, and continuing the medication while nursing is broadly considered compatible with breastfeeding.