Stanford researchers have found that a common drug used to prevent transmission of the HIV virus from mothers to their children may actually spur drug-resistant forms of AIDS.

The drug, nevirapine, is administered as a single pill to the mother before labor and in liquid form to the child just after birth. According to researchers, this persistence of nevirapine levels in the mother’s blood and breast milk puts the mother and baby at risk for developing drug-resistant viral strains, which can be very difficult to treat.

The researchers studied 32 HIV-positive women in Zimbabwe in a study that was originally designed to look at appropriate doses for newborns. In the process, researchers found that drug resistance in the blood persisted for two weeks following administration of nevirapine for over two-thirds of the women. In addition, one-third of the women had drug resistance in their breast milk after eight weeks. Though none of the mothers had drug-resistant forms of HIV, RNA testing two months later showed resistant viruses in the blood of one-third of the mothers as well as in the breast milk of two-thirds.

Researchers also looked at nevirapine levels in search of a correlation with drug resistance in mothers’ breast milk or plasma. However, it was impossible to predict development of resistance because of the different rates of drug metabolism between women.

From their work, researchers developed a concern that infants are prone to infection with a drug-resistant variant during breast-feeding or at some later point after delivery. This puts children at risk even if they escape infection during delivery.

“We know that there are better, more effective modalities to try to prevent infection, such as combination therapies for the mother,” said Dr. Seble Kassaye, Stanford medical fellow and the study’s first author. “We need to continue to work towards using more effective regimens in mothers, both for their own benefit as well as to prevent infection in their babies.”

Researchers have also found that there is an even wider selection of drug-resistance mutations when more sensitive tests, such as clonal analysis, are used.

“What we see with our commonly available tests is not really the full picture of what’s there and what resistance is developing in the mother,” Kassaye said. “From that standpoint, there is probably even more room for concern. But again, in terms of the public health response, the availability of single-dose nevirapine is very valuable because it allows public health departments to actually start addressing the problem — to start building infrastructure around something that seemed doable.”

Researchers are worried, however, about the long-term effects.

School of Medicine Prof. David Katzenstein told U.S. News and World Report that while nevirapine may be better than nothing, the threat of resistance developing on a larger scale could seriously damage future treatment efforts.