To assess whether the timing of post-operative Phosphodiesterase Inhibitor (PDE5i) therapy after Robot-Assisted Radical Prostatectomy (RARP) is associated with a change in early erectile function (EF) outcomes, continence or safety outcomes. Data were prospectively collected from a single surgeon in one tertiary centre. 158 patients were treated with PDE5i therapy post RARP over a 2-year period. PDE5i therapy was started: immediately (day 1-2) post-op in 29%, early (day 3-14) post-op in 37% and late (after day 14) post-op in 34%. EPIC-26 EF scores were collected pre-op and post-op. There were no significant differences in pre-operative characteristics between the therapy groups. Drop in EF scores and percentage return to baseline for unilateral nerve sparing was, respectively, 9 and 11.1% of immediate therapy, 7 and 14.8% of early therapy and 9.7 and 9.5% of late therapy (p = 0.9 and p = 0.6). For bilateral nerve sparing, this was, respectively, 3.5 and 42.9% immediate therapy, 5.5 and 35.5% early therapy and 7.3 and 25% late therapy (p = 0.017 and p = 0.045). Pad free and social continence were achieved in 54% and 37% of those receiving immediate therapy, 60% and 33% for early therapy and 26% and 54% for late therapy. There were no differences in compliance, complication or readmission outcomes. In patients with bilateral nerve sparing RARP, immediate post-operative PDE5i therapy can protect EF and improve early continence outcomes. Therefore, immediate PDE5i therapy should be considered in patients following nerve sparing RARP to maximise functional outcomes.