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Are the Antivirals Acyclovir and Valacyclovir Safe to use for Genital Herpes during Pregnancy

Pregnant women will require treatment with the antivirals acyclovir or valacyclovir for various reasons. These reasons fall into 2 categories. The first is to provide symptomatic relief to the mother. The second category is to prevent neonatal herpes infection, a potentially lethal infection.


Acyclovir is antiviral medication that works by inhibiting viral replication. It is very specific for cells infected by the herpes simplex virus because it requires activation by one of the viral enzymes.

Acyclovir has poor oral bioavailability, about 20%. So it has to be dosed fairly frequently each day.


Valacyclovir is simply a prodrug of acyclovir. This means that it requires hepatic metabolism, but once metabolized works just like acyclovir does.

One benefit of valacylovir is that is has greater bioavailability than acyclovir. So less frequent dosing is required and this may make it easier to take appropriately.

However, one of the draw backs is that valacyclovir is more expensive acyclovir.

Both acyclovir and valacyclovir cross the placenta, but there does not appear to be any concentration of these medications in the fetal tissues.

Acyclovir and valacyclovir are considered to be Pregnancy Category B drugs. No significant teratogenic effects to the fetus have been identified. Potential risks from exposure while in the uterus have been identified. These are potential risks, not known actual risks, that are based on newborns who are undergoing treatment for neonatal herpes. The potential risks to the fetus in utero are:

– Renal insufficiency

– Neutropenia

Valacyclovir and acyclovir are thought to be safe for use in breastfeeding mothers. The breastfeeding infant receives a dose that is only 1% of the effective neonatal dose.

Treatment regimens for Acyclovir and Valacyclovir during pregnancy:

Those pregnant women who are experiencing a symptomatic genital herpes infection may opt to be treated with a one time course of acyclovir or valacyclovir.

A pregnant woman who has a history of severe recurrent genital herpes may opt to go on continuous suppressive anti-viral therapy with acyclovir or valacyclovir.

A pregnant woman may have become infected with herpes for the very first time during the third trimester. In this case, most experts recommend a C-section, whether or not she has signs of infection at the time of labor.

An alternate option is to put the mother on suppressive therapy with acyclovir or valacyclovir. Then type-specific antibodies (which cross the placenta to provide protection to the infant) are checked by the time of delivery.

– If Positive for HSV-2 antibodies a vaginal delivery is possible

– If Negative for HSV-2 antibodies a C-section is required

This alternative option is riskier. The suppressive therapy with acyclovir or valacyclovir may not eliminate viral shedding in the birth canal. Additionally, though antibodies may be present, they may not provide sufficient passive immunity for the infant if the antibody quantities are low.

A pregnant woman who has symptomatic genital herpes at 36 weeks gestational age or has a history of recurrent symptomatic HSV will be put on antiviral suppressive medication at 36 weeks until the baby is delivered.