The expected duration of participation for each participant is 21 weeks. Study participants will be randomly assigned into one of six study groups, each with a different regimen sequence. Each sequence will consist of three study periods and three wash-out periods. Each study period lasts 6 weeks, followed by a 1 week wash-out period. The regimen assigned for a given study period will include either oral TDF, tenofovir vaginal gel, or both. All participants will be prescribed all three regimens in the order designated by their randomized assignment.