The participants will be asked to visit the clinic on 18 occasions, of which three visits involve staying in the unit for approximately 12 hours (day 28, 56 and 84). Blood samples will be taken from the participants throughout the study to measure the levels of Descovy, Viread and Rifadin in blood. Blood and urine will also be collected throughout the study for safety analysis. This is to ensure the participants are healthy to take part or to continue taking part in the study. The total amount of blood collected from each participant during the study will be approximately 590ml (approximately a pint).