The purpose of this study is to look at the levels of three HIV medications: Descovy®, Viread® and Rifadin® in the blood after drug intake has been stopped, in order to understand how long these drugs persist in the blood. The study will specifically look at blood levels of these three drugs after taking them every day for 28 days. Participants will take Descovy® on a first stage, a combination of Descovy® and Rifadin® on a second stage, and Viread® on a third stage. If the participants decide to take part, the duration of the study will be up to 85 days plus a screening visit which will take place up to 28 days prior to the start of the study, and a follow up visit, which takes place 28 to 36 days after the last dose of study medication. This study is not randomised which means that all participants will receive all study medications in the same order. The participant and the study doctor will know which study medications the participant is taking at all times during the study.