Breastfeeding mothers can safely take antiretroviral medicine to prevent HIV infection without posing a risk to their babies, according to a new study from the University of Washington School of Public Health.

Pre-exposure prophylaxis, or PrEP, is a way for people who do not have HIV—but who are at very high risk of getting it—to prevent infection by taking a daily antiretroviral drug containing tenofovir and emtricitabine. When taken consistently, PrEP has been shown to significantly reduce the risk of HIV infection for women, study authors noted. However, little is known about the safety of using antiretrovirals during pregnancy and while breastfeeding.

“This study provides important new empirical evidence strongly suggesting that tenofovir-based PrEP can safely be given to breastfeeding women without putting their infants at increased risk of adverse effects,” said lead author  Kenneth Mugwanya, a Ugandan physician who earned a PhD in epidemiology from the School and is a senior fellow in the Department of Global Health.

The study, published online Sept. 27 in PLOS Medicine, found that infants had very low exposure to antiretroviral drugs through breastmilk. In fact, the drug tenofovir was undetectable in 94 percent of the infants studied, researchers noted.

To determine the extent to which infants absorb PrEP medication through breastmilk, researchers administered oral antiretroviral drugs to 50 HIV-negative, nursing mothers in Kenya and Uganda for 10 days. Researchers then measured drug concentrations in maternal blood and breastmilk, and they obtained a single blood sample from the 50 corresponding babies, who were one to 24 weeks old.

Findings showed the estimated doses of tenofovir and emtricitabine a breastfeeding infant would ingest each day were less than 0.01 percent and 0.5 percent, respectively, compared to the doses an infant would receive if they were undergoing treatment for HIV.

“This study will advance effective HIV prevention in young women, particularly those in regions with high HIV burden and where fertility is high,” Mugwanya said.

Mugwanya co-authored the study with colleagues from the School of Public Health: Nelly Mugo, Kenneth Ngure, Grace John-Stewart, Andy Stergachis, Connie Celum and Jared Baeten. The study was conducted in collaboration with Johns Hopkins University, Makerere University in Uganda and Partners in Health and Development.