Patients successfully completing screen will be randomly assigned to one of four treatment groups at Visit 2 (V2): placebo or one of 3 active drug doses of DA-8159 tablets. Patients will be given medication for at-home use and will also be given diaries for recording information regarding sexual encounters, quality of erections and adverse events. At 4-week intervals for 3 months (V3, V4, V5), patients will return to the clinic to review and collect SEP diaries, safety data, 12-lead ECG and medication reconciliation. In addition, at each 4-week interval patients will complete an IIEF, and at Visit 5 will have blood drawn for safety evaluation.