A Study of Tadalafil and Sildenafil in Men With Erectile Dysfunction in China

Inclusion Criteria: Study Subjects * Are able to read, understand and provide signed informed consent. * Have a history ED of any etiological classification (psychogenic, organic, or mixed) and any severity for at least 3 months. * Have never taken any phosphodiesterase 5 (PDE5) inhibitor for the treatment of ED. * Anticipate having the same female sexual partner during the study for recording responses to efficacy questionnaires. If a qualifying participant has more than one female partner during the study, the participant will not be excluded from the trial. However, the participant will be required to respond to the questionnaires based on his sexual interactions with only one of these partners. * Make at least 4 sexual intercourse attempts, with the female sexual study partner, during the 4-week run-in period and during the final 4 weeks of each 8-week treatment period. * Not use any ED treatment including the use of herbal therapy and traditional Chinese medicine (TCM) for the treatment of ED other than study medication at any time during the study and for 96 hours after study completion. Partners * Are female, at least 18 years of age at screening and will have the same male study subject as her sexual partner during the study. * Are able to read, understand and provide signed informed consent. * Agree to make at least 4 sexual intercourse attempts with the male sexual study partner during the 4-week run-in phase and during the final 4 weeks of each 8-week treatment period. * Willing to participate in recording responses to efficacy questionnaires, sexual quality of life questionnaires and other instruments used in this study. Exclusion Criteria: * Present with ED caused by other primary sexual disorders including premature ejaculation or ED caused by untreated endocrine disease. * Have a history of radical prostatectomy, or other pelvic surgery with subsequent failure to achieve erection. * Have a history of penile implant. * Have a clinically significant penile deformity in the opinion of the investigator. * Exhibit evidence of clinically significant renal insufficiency as determined by the investigator, or defined as receiving renal dialysis or having an estimated creatinine clearance of less than (\<)30 milliliters per minute (mL/minute) at screening, as calculated by the local laboratory using the Cockcroft-Gault formula. * Exhibit evidence of active symptomatic hepatobiliary disease at Visit 1. * Exhibit Hemoglobin A1c greater than (\>) 11 percent (%) at screening, in patients with a history of diabetes mellitus of any type. * Present with chronic stable angina treated with long-acting nitrates, or with chronic stable angina requiring short-acting nitrates in the last 90 days, or with angina occurring during sexual intercourse in the last 6 months. * Have met the criteria for unstable angina within 6 months before screening, or have a history of myocardial infarction or coronary artery bypass graft surgery within 90 days before screening, or percutaneous coronary intervention within 90 days before screening. * Have any supraventricular arrhythmia with an uncontrolled ventricular response (mean heart rate \>100 bpm) at rest, or have any history of spontaneous or induced sustained ventricular tachycardia (heart rate \>100 beats per minute (bpm) for greater than or equal \[≥\] to 30 seconds) despite medical or device therapy, or use an internal cardioverter-defibrillator. * Have a history of sudden cardiac arrest despite medical or device therapy. * Exhibit any evidence of congestive heart failure within 6 months before screening. * Have had a new or significant cardiac conduction defect within 90 days before screening. * Exhibit systolic blood pressure \>170 or \<90 milliliters of mercury (mm Hg) or diastolic blood pressure \>100 or \<50 mm Hg at screening, or have a history of malignant hypertension. * Have retinitis pigmentosa. * Have a history of significant central nervous system injuries (including stroke and spinal cord injury) within the last 6 months. * Have a history of human immunodeficiency virus (HIV) infection. * Have a condition that in the opinion of the investigator would interfere with the patient's ability to provide informed consent or comply with study instructions, would place the patient at increased risk, or might confound the interpretation of study results. * Currently receive treatment with nitrates, alpha \[1\]-adrenergic blockers (such as doxazosin), cancer chemotherapy, or antiandrogens (except finasteride taken as Propecia™ or Proscar® or Avodart® \[dutasteride\]). * Have a history of drug, alcohol, or substance abuse within the past 6 months, as assessed by the investigator. * Have received treatment within the last 30 days with a drug that has not received regulatory approval for any indication at the time of study entry. * Patients who are currently taking TCM for ED treatment within the last 30 days prior to study entry or planned concomitant administration of TCM for ED treatment during study enrolment. * Are currently enrolled in, or discontinued with the last 30 days from a clinical trial involving an investigational product or unapproved use of a drug or device, or concurrently enrolled in any other type of medical research judged not to be scientifically or medically compatible with this study. Patients who have been screen failures in other studies may be eligible if they satisfy the inclusion/exclusion criteria. * Have a history of loss of vision in one eye because of non-arteritic anterior ischemic optic neuropathy (NAION), regardless of whether this episode was in connection or not with previous PDE5 inhibitor exposure. * Have rare hereditary problems of galactose intolerance, the Lapp lactase deficiency or glucose-galactose malabsorption lactose intolerance. * Are currently in a sexual relationship with a female of child-bearing potential where no form of birth control is being used. A female of child-bearing potential is a female who is not surgically or chemically sterilized and who is between menarche and 1-year post menopause. * Are currently in or are planning to be in a sexual relationship with a pregnant female or are currently in a relationship where either partner is actively trying to conceive.