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Purpose: 5-alpha reductase inhibitor (5ARI) reduces prostate-specific antigen (PSA) by half but its effect on prostate health index (phi) is unknown. This study aims to investigate this effect and to enable accurate interpretation of phi in men with elevated PSA and on 5ARI., Methods: This is a prospective study evaluating the effect of finasteride on PSA, free PSA (fPSA), [ - 2]proPSA (p2PSA) and phi at 6 and 12 moths in men with PSA 4-20 ng/mL, no prior 5ARI use, and one negative prostate biopsy within 6 months before recruitment. The 5ARI Finasteride (5 mg/day) for 1 year was offered if International Prostatic Symptom Score (IPSS) was ≥ 8 at baseline. 5ARI group included patients taking finasteride, while control group included patients not on finasteride. The blood results were compared with t-test between baseline and different time points in each group and between groups at 1 year., Results: 164 men fit the inclusion criteria and 150 were analyzed. In 5ARI group (n = 100) at 1 year, mean PSA reduced by 51.4% from 8.9(± SD 3.7) to 4.4(± SD 2.8)ng/mL (paired t-test, p < 0.001), fPSA reduced by 52.4% from 1.6(± 0.6) to 0.8(± 0.4)ng/mL (p < 0.001), p2PSA reduced by 55.3% from 18.4(± 8.8) to 8.3(± 5.6)pg/mL (p < 0.001), and phi reduced by 34.2% from 33.7(± 11.9) to 22.4(± 12.5) (p < 0.001). PSA and phi values in the control group remained static over 1 year and significantly higher than those in 5ARI group., Conclusion: This study demonstrated p2PSA and phi are reduced by about 55% and 34% in men on 5ARI. A conversion factor of division by 0.66 is needed for phi in men on finasteride to allow the interpretation and use of phi in men on 5ARI., (© 2023. The Author(s), under exclusive licence to Springer Nature B.V.)