OBJECTIVE: To assess the effectiveness of oral spironolactone for acne vulgaris in adult women. DESIGN: Pragmatic, multicentre, phase 3, double blind, randomised controlled trial. SETTING: Primary and secondary healthcare, and advertising in the community and on social media in England and Wales. PARTICIPANTS: Women (≥18 years) with facial acne for at least six months, judged to warrant oral antibiotics. INTERVENTIONS: Participants were randomly assigned (1:1) to either 50 mg/day spironolactone or matched placebo until week six, increasing to 100 mg/day spironolactone or placebo until week 24. Participants could continue using topical treatment. MAIN OUTCOME MEASURES: Primary outcome was Acne-Specific Quality of Life (Acne-QoL) symptom subscale score at week 12 (range 0-30, where higher scores reflect improved QoL). Secondary outcomes were Acne-QoL at week 24, participant self-assessed improvement; investigator's global assessment (IGA) for treatment success; and adverse reactions. RESULTS: From 5 June 2019 to 31 August 2021, 1267 women were assessed for eligibility, 410 were randomly assigned to the intervention (n=201) or control group (n=209) and 342 were included in the primary analysis (n=176 in the intervention group and n=166 in the control group). Baseline mean age was 29.2 years (standard deviation 7.2), 28 (7%) of 389 were from ethnicities other than white, with 46% mild, 40% moderate, and 13% severe acne. Mean Acne-QoL symptom scores at baseline were 13.2 (standard deviation 4.9) and at week 12 were 19.2 (6.1) for spironolactone and 12.9 (4.5) and 17.8 (5.6) for placebo (difference favouring spironolactone 1.27 (95% confidence interval 0.07 to 2.46), adjusted for baseline variables). Scores at week 24 were 21.2 (5.9) for spironolactone and 17.4 (5.8) for placebo (difference 3.45 (95% confidence interval 2.16 to 4.75), adjusted). More participants in the spironolactone group reported acne improvement than in the placebo group: no significant difference was reported at week 12 (72% v 68%, odds ratio 1.16 (95% confidence interval 0.70 to 1.91)) but significant difference was noted at week 24 (82% v 63%, 2.72 (1.50 to 4.93)). Treatment success (IGA classified) at week 12 was 31 (19%) of 168 given spironolactone and nine (6%) of 160 given placebo (5.18 (2.18 to 12.28)). Adverse reactions were slightly more common in the spironolactone group with more headaches reported (20% v 12%; p=0.02). No serious adverse reactions were reported. CONCLUSIONS: Spironolactone improved outcomes compared with placebo, with greater differences at week 24 than week 12. Spironolactone is a useful alternative to oral antibiotics for women with acne. TRIAL REGISTRATION: ISRCTN12892056.