BACKGROUND/PURPOSE(S): Data regarding the concentrations of tenofovir-diphosphate (TFV-DP) and emtricitabine-triphosphate (FTC-TP) in the Asian population receiving pre-exposure prophylaxis (PrEP) for HIV with tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) (TDF/FTC) are limited, and the associations between the frequency of TDF/FTC administration and drug concentration among people receiving on-demand PrEP remain unclear. METHODS: Fifty-seven participants receiving daily TDF/FTC and 113 participants receiving on-demand TDF/FTC were enrolled in this study. The concentrations of TFV-DP and FTC-TP were measured in dried blood spots using liquid chromatography‒mass spectrometry. RESULTS: Thirty-six (62.2 %) daily PrEP users and 38 (33.6 %) on-demand PrEP users achieved TFV-DP concentrations ≥700 fmol/punch. Higher proportions of undetectable FTC-TP were observed in participants whose TFV-DP concentrations were ≤350 fmol/punch, regardless of the frequency of TDF/FTC administration. In participants who used on-demand PrEP, the TFV-DP and FTC-TP concentrations were moderately correlated with the TDF/FTC tablets taken when sampling was performed within 12-24 h after the last dose of TDF/FTC (R = 0.63, p = 0.006 and R = 0.75, p = 0.0005). In addition, on-demand PrEP users who had received 8 tablets within the last 28 days had a median TFV-DP concentration similar to that of those participants who had received 16 tablets (544.6 vs. 556.9 fmol/punch, p >
0.99). CONCLUSIONS: These results underscore the importance of well-controlled sampling times for obtaining reliable TFV-DP and FTC-TP concentrations to estimate the adherence and effectiveness of on-demand PrEP.