Inclusion Criteria:
* male 19-65 years of age;
* Available to participate for the planned duration of the study;
* Able and willing to complete the informed consent process;
* Agree to have blood samples collected and stored for the study;
* Agree not to use approved or experimental benign prostatic hyperplasia or erectile dysfunction treatments anytime during the course of the study;
* Have not taken finasteride or dutasteride therapy, any other lower urinary tract symptom (LUTS) therapy or phosphodiesterase type 5 (PDE5) inhibitors for specified duration of time prior to the study;
* Have not taken nitrates, alpha blockers, antihypertensives, alcohol, CYP3A4 inhibitors and CYP3A4 inducers for a specified duration of time prior to the study;
* Have a prostate specific antigen (PSA) score within acceptable range defined for study or negative biopsy of the prostate for cancer within 12 months of the study.
Exclusion Criteria:
* A condition in which repeated blood draws poses more than minimal risk for the subject such as hemophilia, other severe coagulation disorders or significantly impaired venous access;
* A condition that requires active medical intervention or monitoring to avert serious danger to the subject's health or well-being;
* subjects with dentures, partial dentures or braces, subjects with swallowing disorders and subjects who abuse drugs, alcohol or tobacco;
* Currently taking nitrates or nitro compounds, alpha blockers, antihypertensives, alcohol, CYP3A4 inhibitors, CYP3A4 inducers;
* Currently taking any medicines known to conflict with tadalafil;
* History of urinary retention or lower urinary tract (bladder) stones 6 months before the start of the study;
* History of bladder outlet obstruction or urethral obstruction due to stricture, valves, sclerosis, or tumor;
* History of diabetes;
* History of cardiac conditions including angina requiring certain treatment with nitrates, heart disease or coronary conditions including myocardial infarction, bypass surgery, angioplasty or stent placement for a specified time before starting the study;
* Certain neurological conditions associated with bladder problems or injuries to the brain or spinal cord within a specified time before starting the study.
