A drug known as nevirapine, which is commonly used in the developing world to prevent transmission of HIV from mother to child can persist in the breast milk and blood of the mother, researchers have found.

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.