We investigated the determinants of daily PrEP use and coverage of condomless anal sex (CAS) by PrEP among men who have sex with men in Wales, UK. We measured PrEP use by electronic monitors and CAS by secure online surveys. We defined PrEP use based on daily medication cap openings and coverage as CAS episodes preceded by ≥ 3 days of PrEP use and followed by ≥ 2 days of PrEP use. We included 57 participants (5463 observations). An STI diagnosis was associated with lower PrEP use but also lower PrEP coverage. Older adults had higher PrEP use. A belief that other PrEP users took PrEP as prescribed was associated with lower PrEP coverage. An STI diagnosis is an important cue for an intervention, reflecting episodes of high-risk sexual behaviour and low PrEP coverage. Other results provide a basis for the development of an evidence-informed intervention for promoting coverage of PrEP.