Premature ejaculation (PE) and erectile dysfunction (ED) are forms of sexual dysfunction in men. An objective measurement of PE in clinical studies is the intravaginal ejaculatory latency time (IELT), which is the time it takes for a man to ejaculate during sexual intercourse (as measured by stopwatch). This is a multicenter, double-blind (neither the physician or the study participant will know the identity of the treatment assigned), randomized (study drug assigned by chance) efficacy and safety study of dapoxetine compared with placebo (a sugar pill) in men with premature ejaculation who are currently being treated for ED with a phosphodiesterase 5 (PDE-5) inhibitor such as sildenafil, vardenafil, or tadalafil. A maximum of 656 men 18 years or older (hereafter referred to as study participants) who have received treatment with a PDE-5 inhibitor for at least 3 months prior to study entry will be enrolled. The study will last approximately 18 weeks and includes a 4-week screening period, a 12-week treatment period, and a follow-up telephone contact approximately 2 weeks after the end of treatment. Both the study participant and his partner will be required to attend the screening visit and to sign an informed consent form documenting that they understand and agree to the requirements for the study. After initial screening procedures are completed, study participants who qualify for the study will enter a 4-week screening period. During the 4 weeks, the study participant and his partner will be provided with a stopwatch to time and record the IELT during all attempts at sexual intercourse. At the next scheduled clinic visit which is Day 1 of the double-blind treatment period, study participants who continue to qualify for the study will be assigned by chance (like flipping a coin) to receive 1 of 2 study treatments (dapoxetine or placebo) for 12 weeks in addition to prescribed treatment with a PDE-5 inhibitor. Study participants will be instructed to take study drug with or without food with at least 1 full glass of water approximately 1 to 3 hours before sexual activity (no more than 1 dose should be taken within a 24-hour period). During the 12-week treatment period, the study participant and his partner will be asked to time and record the IELT during all attempts at sexual intercourse on Treatment Event Logs provided. Study participants will return to the clinic after 4, 8 and 12 weeks of treatment for routine safety assessments (including review of Treatment Event Logs returned) and to be dispensed study drug. Following 12 weeks of treatment (or at the time of early withdrawal from the study) end-of-treatment safety and efficacy evaluations will be performed at the final clinic visit. Approximately 2 weeks later, a follow up telephone call will be made to the study participant to collect information on any adverse events that may have occurred or concomitant therapy received since the time of the last clinic visit. The primary outcome measure in the study is the average IELT, as measured by stopwatch, during sexual intercourse at the end of the treatment period (Week 12). Safety will be monitored during the study by evaluating adverse events, physical examination findings, results from clinical laboratory tests, and concomitant medication usage. An Independent Data Monitoring Committee (IDMC) will be established to monitor the safety and efficacy of study participants during the study. In addition, an interim (preliminary) analysis will be performed during the study to monitor safety and efficacy after approximately 268 men have completed 12 weeks of treatment (also includes any study participants who did not complete treatment and were withdrawn early from the study). Study participants will receive either dapoxetine or matching placebo tablets at a dose of 30 mg prn (as needed) taken orally (by mouth) with or without food with at least 1 full glass of water approximately 1 to 3 hours before sexual activity (not to be taken more than once every 24 hours). At Weeks 4 or 8, the dose of dapoxetine or matching placebo may be increased to a maximum of 60 mg prn if specific predefined criteria are met or be subsequently decreased from 60 to 30 mg at Weeks 4 or 8.