• Psychosocial outcomes, as measured by Sexual Self-Confidence, Spontaneity, and Time Concerns domains of PAIRS. [ Time Frame: 26 weeks (including two washout periods of 1 week each) ] [ Designated as safety issue: No ]
• Efficacy, as measured by the Erectile Function domain (sum of Items 1 through 5 and Item 15) of the International Index of Erectile Function (IIEF) and the frequency of morning erection. [ Time Frame: 26 weeks (including two washout periods of 1 week each) ] [ Designated as safety issue: No ]
• Sexual satisfaction, as measured by the Intercourse Satisfaction and Overall Satisfaction domains of the IIEF. [ Time Frame: 26 weeks (including two washout periods of 1 week each) ] [ Designated as safety issue: No ]
• Treatment satisfaction, as measured by the Erectile Dysfunction Inventory of Treatment Satisfaction (EDITS). [ Time Frame: 26 weeks (including two washout periods of 1 week each) ] [ Designated as safety issue: No ]
• Safety and tolerability. [ Time Frame: 26 weeks (including two washout periods of 1 week each) ] [ Designated as safety issue: Yes ]
• Psychosocial outcomes, as measured by the Self-Esteem And Relationship (SEAR) questionnaire; and Questions 1 to 4 of the Patient Perception and Feelings Questions (PPF-Q). [ Time Frame: 26 weeks (including two washout periods of 1 week each) ] [ Designated as safety issue: No ]

The primary purpose of the study is to help answer whether tadalafil taken once a day can help improve the psychological outcomes (such as sexual self confidence, spontaneity and time concerns) compared to sildenafil taken as needed in patients with erectile dysfunction.

• Drug: tadalafil
• Drug: sildenafil citrate

• Experimental: Tadalafil 5 mg OaD, sildenafil citrate 100 mg PRN, tadalafil 20 mg PRN
• Experimental: Tadalafil 5 mg OaD, tadalafil 20 mg PRN, sildenafil citrate 100 mg PRN
• Experimental: Sildenafil citrate 100 mg PRN, tadalafil 5 mg OaD, tadalafil 20 mg PRN
• Experimental: Sildenafil citrate 100 mg PRN, tadalafil 20 mg PRN, tadalafil 5 mg OaD
• Experimental: Tadalafil 20 mg PRN, tadalafil 5 mg OaD, sildenafil citrate 100 mg PRN
• Experimental: Tadalafil 20 mg PRN, sildenafil citrate 100 mg PRN, tadalafil 5 mg OaD

Inclusion Criteria:

• History of erectile dysfunction.
• Have had satisfactory response to tadalafil, sildenafil citrate, or vardenafil HCl for a period of at least 6 months and expect frequent use (at least one dose per week, on average) when administered as needed.
• Have never used tadalafil 5 mg or 2.5 mg once a day therapy.
• Anticipate having the same adult female sexual partner willing to participate during the study.
• Agree to make at least four sexual intercourse attempts during the first four weeks of the study.
• Agree not to use any other treatment for ED (even herbal treatments) during the study.
• Agree to follow the directions given by the study doctor and staff about using the study drug.

Exclusion Criteria:

• Have any other primary sexual disorders present or penile deformity.
• Have history of radical prostatectomy or penile implant.
• Have problems with your kidneys, liver, or nervous system.
• Have uncontrolled diabetes, as measured by a blood test for haemoglobin A1C greater than a value of 11%.
• Are being treated for heart disease with any drug that is called a nitrate (for example, nitroglycerin).
• Have chest pain (called unstable angina or angina) that requires treatment.
• Have heart disease that causes symptoms after you exert yourself.
• Have had any of the following in the past 90 days: Heart attack, also known as a myocardial infarction (MI); Heart bypass surgery (called coronary artery bypass graft surgery); Had a procedure to open up blood vessels in the heart known as angioplasty or stent placement (percutaneous coronary intervention).
• Have a history of loss of vision in one eye because of nonarteritic anterior ischemic optic neuropathy (NAION).
• Have retinitis pigmentosa.
• Have history of HIV.
• Have very high or very low blood pressure (your study doctor will discuss the limits with you).
• Have had a stroke or a significant injury to your brain or spinal cord within the last 6 months.
• Have rare hereditary problems of galactose intolerance, the Lapp lactase deficiency or glucose-galactose malabsorption Lactose intolerance.
• Have a scheduled or planned surgery requiring anaesthesia during the course of the study.
• Have a scheduled cataract surgery during the curse of the study.