The primary objective of this study is to define the mean, variance, and dose proportionality for tenofovir-diphosphate(TFV-DP) in dried blood spots resulting from 33%, 67%, and 100% of daily dosing with 200mg emtricitabine and 300mg of tenofovir disoproxil fumarate (as Truvada®). With this information, a model will be established to predict adherence rates to TFV-DP using DBS. Forty-eight healthy HIV-uninfected adult participants who are at low risk for HIV infection will be randomized to one of 6 sequences consisting of two directly observed dosing regimens, 33%/67%, 33%/100%, 67%/33%, 67%/100%, 100%/33%, and 100%/67% with each dose regimen lasting approximately 12 weeks, separated by an approximately 12 week washout period. DBS will be collected at regular intervals, including during the washout. The hypothesis of the study is that levels of TFV-DP in DBS will predict adherence rates in the preceding 1-3 months.