OBJECTIVES: This study systematically reviews existing evidence to determine the efficacy and adverse events of dutasteride, alone or in combination, versus a placebo or control, used for the treatment of benign prostatic hyperplasia (BPH). METHODS: Medline and the Cochrane Library were searched for randomized controlled trials that were longer than six months in duration. The study population was men, aged 50 or older, with moderate and severe symptoms of BPH as determined by the International Prostate Symptom Score (IPSS) and clinical parameters including maximum urinary flow rate (Qmax) and prostate volume. A meta-analysis with a random effects model was conducted to synthesize the findings of multiple studies. Heterogeneity among the studies was assessed both graphically with forest plots and statistically with I2 statistics. RESULTS: Dutasteride was superior to the placebo in reducing urinary symptoms measured by IPSS [weighted mean difference (WMD) = -2.50, 95% confidence intervals (CIs) = -2.91 to -2.09], prostate volume (WMD = -12.90, 95% CIs = -14.27 to -11.53) and in increasing Qmax (WMD = 1.30, 95% CIs = 0.99 to 1.61). However, risk ratios for any adverse events (1.27, 95% CIs = 0.99 to 1.62) and for drug-related adverse events (1.47, 95% CIs = 0.77 to 2.79) were not significantly different between those treated with dutasteride and those in the control group. When dutasteride was administered with tamsulosin, the reduction in IPSS scores and the increase in Qmax were greater than if the medications were administered alone, although prostate volume changed little. For both BPH-related acute urinary retention (AUR) and BPH-related surgery, no significant difference in risk ratio was observed between the combination group and the dutasteride group.  CONCLUSIONS: Dutasteride can be used for those with moderate to severe BPH symptoms when benefits from efficacy are considered to be greater than the loss from adverse events.