The purpose of this study is to assess the pharmacodynamics (the actions or effects a drug has on the body), pharmacokinetics (blood levels), safety, and tolerability of dapoxetine 60 mg when concomitantly administered in participants taking terazosin.

This is a multiple-center, randomized (participants are assigned to study drug by chance), multiple-dose, double-blind (neither physician nor participant knows the treatment assigned), placebo (inactive substance that is compared with a drug to test whether the drug has a real effect in a clinical trial)-controlled, 2-period crossover (participants may receive different interventions sequentially during the trial) study to assess the pharmacodynamics, pharmacokinetics, safety, and tolerability of dapoxetine when concomitantly administered in adult male participants who have been on a stable dose of terazosin or doxazosin for at least 6 weeks. Study drugs used will be terazosin (a drug given for the treatment of hypertension and benign prostatic hyperplasia) and dapoxetine (a new drug being studied for the treatment of premature ejaculation). This study consists of a screening phase followed by a double-blind, placebo-controlled treatment phase consisting of 2 treatment periods. Participants will remain on their prescribed dose of terazosin and participants taking doxazosin will be converted to terazosin using the protocol-defined dose conversion (including assessment of control of urinary symptoms and risk for urinary retention). Participants will be administered concomitant (at the same time) dapoxetine or placebo once daily. Blood samples for pharmacodynamic and pharmacokinetic measurements will be collected at selected times during the study. Safety will be monitored. The total duration of study participation will be approximately 49 days. Participants will remain on their prescribed dose of terazosin (2 to 10 mg taken once daily by mouth) and will be administered daily by mouth concomitant dapoxetine 60 mg or placebo doses during 2 treatment periods (each 7 days in duration conducted in the study center). A period of up to 14 days will separate the treatment periods.

Inclusion Criteria:

• On a stable regimen of terazosin (2 to 10 mg) or doxazosin (2 to 8 mg) taken daily for at least 6 weeks for the treatment of hypertension or benign prostatic hyperplasia
• a body mass index between 18 and 35 kg/m², inclusive
• a body weight of not less than 50 kg
• supine (laying down) blood pressure measurements between 90 and 150 mmHg, inclusive
• and, diastolic blood pressure measurements no higher than 95 mmHg.

Exclusion Criteria:

• History of current clinically significant illness or any other illness that the investigator considers should exclude the study participant or that could interfere with the interpretation of study results
• symptomatic orthostatic hypotension (a decrease of >=20 mm Hg systolic blood pressure measured after 2 but before 3 minutes after changing from supine (laying down) to standing position)
• taking a medication that is known to cause orthostatic hypotension, other than terazosin
• and, taking more than 2 other antihypertensive medications or taking an antihypertensive medication that is excluded.