Experts at the Stanford University School of Medicine say that the persistence of the drug in mothers' milk may put them and their babies at the risk of developing drug-resistant strains of the virus.
The researchers say that nevirapine stays in the blood and breast milk of the infected mothers for at least two weeks, and that the virus may transform itself into drug resistant strains of HIV within that duration.
"In the short term, nevirapine is better than nothing. But in the long term, I'm concerned about conferring resistance. If you're talking about resistance on a broad scale, it could jeopardize future treatment for mothers and infants," said Dr. David Katzenstein, professor of infectious diseases and principal investigator of the study.
It was found in some previous studies that as many as 69 percent of HIV-positive mothers and as many as 80 percent of babies born infected, even after being given a single-dose of nevirapine without AZT, may develop nevirapine-resistant strains of the virus.
Dr. Katzeinstein said that his team undertook the latest study in order to better understand this problem.
He and his colleagues looked at a group of 32 HIV-positive pregnant women in Zimbabwe, the sub-Saharan African country that has been hard hit by the virus, with an estimated 17 to 18 percent of young adults estimated to be infected.
None of the women had been treated for their HIV at the time of the study. The only drug they received was the single dose of nevirapine when they went into labour, largely for the sake of their babies.
The researchers found that the drug persisted in the body for weeks, with more than half of the women having detectable levels in their blood within two weeks after delivery.
They said that two-thirds had measurable levels in their breast milk at two weeks.
Although none of the HIV-infected women carried drug-resistant strains of the virus at the outset of the study, at two months after birth RNA tests showed a third of them had drug-resistant virus in their blood.
Sixty-five percent were even found to have drug-resistant strains in their breast milk, with the potential to pass this on to their babies through breastfeeding.
Dr. Seble Kassaye, instructor in infectious diseases and first author of the study, said that the mothers who were most likely to develop resistant virus were those whose disease was more advanced as indicated by lower CD4 cell counts, the immune cells targeted by HIV.
She said that with advanced HIV infection, such women were likely have more replicating virus, so they might be more prone to developing mutations that make the virus resistant to treatment.
She further said that the women might be less likely to develop the hard-to-treat strains later if the had access to better, combination antiretroviral treatment to optimally suppress virus replication.
"It reinforces the need to treat these women with combination therapy, thereby providing better prevention for the infant, while providing better treatment for the mother. Public health efforts should continue to expand combination therapy so that mothers and babies aren't left vulnerable to drug resistance," Kassaye said.
A presentation on the study was made at the International AIDS Conference in Mexico City.