Endothelial Markers and Tadalafil Response

Inclusion Criteria: * Participants must fulfill all of the following: * Men aged 30 to 70 years. * ED lasting at least 3 months. * IIEF-5/SHIM score of 21 or less or clinically documented ED. * Clinical features suggesting predominantly vasculogenic ED. * Sexually active or willing to attempt sexual intercourse during the study. * Able to complete the study questionnaires. * Willing to take tadalafil 5 mg once daily for 12 weeks. * Able to provide written informed consent. * Willing to avoid other ED treatments during the study unless medically necessary. Exclusion Criteria: * Patients will be excluded if they have: * ED primarily attributed to psychogenic causes. * Predominantly neurogenic ED. * Severe untreated hypogonadism likely to be the primary cause of ED. * Untreated hyperprolactinemia or another major endocrine disorder. * Significant penile anatomical abnormality interfering with intercourse, including severe Peyronie's disease. * Previous radical pelvic surgery or pelvic radiotherapy. * Active urinary or genital infection. * Unstable angina or unstable cardiovascular disease. * Recent myocardial infarction or stroke. * Uncontrolled hypertension or clinically significant hypotension. * Severe renal impairment requiring an alternative tadalafil regimen. * Severe hepatic impairment. * Active malignancy or expected survival of less than one year. * Major psychiatric illness, cognitive impairment or inability to comply with follow-up. * Current use of another PDE5 inhibitor or intracavernosal therapy that cannot be discontinued. * Concomitant use of organic nitrates, nitric oxide donors or nicorandil. * Known hypersensitivity to tadalafil. * Participation in another study that may affect erectile function or vascular measurements. * Refusal or inability to provide informed consent.