Gilead Sciences has begun a major trial in Western nations to compare how well Descovy (emtricitabine/tenofovir alafenamide, or TAF) works as pre-exposure prophylaxis (PrEP) against HIV compared with Truvada (tenofovir disoproxil fumarate, or TDF/emtrici-tabine). The trial of 5,000 at-risk transgender women and cisgender men who have sex with men will also compare the drugs’ effects on bone mineral density, kidney function and other safety measures. The first results are expected in early 2019; the study should be completed in late 2020. The FDA approved Descovy in April 2016 for use as HIV treatment. Research among HIV-positive people has shown that the drug is less toxic to the bones and kidneys than Truvada. The reason is that the new version of tenofovir in Descovy is safer than the older one in Truvada.