Nevirapine is already widely used to protect babies at birth. A single dose given to the mother as she goes into labor and to the baby at birth cuts transmission by 47 percent.
Researchers wanted to see if they could safely continue giving the drug to babies for as long as six weeks.
In many developing countries breast-feeding is the only option.
Nevirapine is sold under the brand name Viramune by privately held Boehringer Ingelheim.