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Objective: To assess the association between 5α‐reductase inhibitor (5‐ARI) use and high grade (Gleason score 8–10) prostate cancer. Patients and Methods: We conducted a population‐based nested matched case–control study using the French national health insurance database linked to data from all pathology laboratories in Brittany, France. Among 74 596 patients with ≥1 drug reimbursement for symptomatic benign prostate hypertrophy (BPH) between 1 January 2010 and 31 December 2011, 767 incident prostate cancer cases between 1 January 2012 and 31 December 2013 were matched according to age and delay between the first observed delivery of drug for BPH (5‐ARIs, α‐blockers or phytotherapy) and diagnostic date of the case to five control patients, using an incidence density sampling design. Results: A total of 963 patients (153 cases, 810 controls) had been exposed to 5‐ARIs. A significant heterogeneity (P = 0.005) was detected across cancer grades when estimating the association between prostate cancer and long‐term (≥2 years) 5‐ARI use vs no 5‐ARI exposure: adjusted conditional odds ratio 1.76 (95% confidence interval [CI] 0.97–3.21) for Gleason score ≥8 and 0.64 (95% CI 0.44–0.93) for Gleason score < 8. Conclusion: Our results indicate an increased risk of high grade and a decreased risk of low grade prostate cancer associated with 5‐ARI use. Patients treated for >2 years with 5‐ARIs should be informed about the increased risk of development of high grade disease. [ABSTRACT FROM AUTHOR]