Placebo-controlled randomized clinical trials (RCT) are the gold standard to provide applicable evidence for clinical practice. A vast number of RCTs for treatment of several disorders has shown a mean placebo effect of 30%. Erectile dysfunction (ED) is a high prevalent disease and its first-line therapy is oral phosphodiesterase type-5 inhibitor (iPDE5) (i.e., sildenafil, vardenafil and tadalafil). Although the placebo effect in iPDE5 RCT occurred at a rate as high as 50%, in the last decade, with the revolutionary discovery of effective pharmacotherapy for ED, oral iPDE5 has become one of the most common prescribed drugs for men. The objective of this study is to evaluate the influence of patients' awareness on the composition of the therapeutic drug in the outcomes of the oral treatment for ED.

Inclusion Criteria:

• 18 years of age or older
• Complaint of ED for at least 6 months prior the start of the study, according to medical history and to the questionnaires IIEF, SEAR and QEQ.
• A stable heterosexual relationship.
• To be in good health prior the participation in the study, with no important clinical abnormalities, determined by: clinical history, physical examination, blood chemistry and hematology, urinalysis (If the standard values of the laboratory for these tests are not evidenced, subjects can be included if the researcher consider the changes as not clinically significant).

Exclusion Criteria:

• History of significant cardiovascular, pulmonary, gastrointestinal, hematological, neurological (including epilepsy), locomotor, immunologic, ophthalmic, metabolic, endocrine, thromboembolic, rheumatologic, cancer, kidney or liver disease.
• Diagnosis of psychiatric conditions, e.g. depression, anxiety, dysthymia, mania, panic, agoraphobia, social phobia, suicidal ideation, obsessive-compulsive disorder, post-traumatic stress, psychotic disorders, schizophrenia, alcoholism, dependence on psycho-active substances.
• History of HIV, hepatitis B or hepatitis C.
• Hyperprolactinemia or untreated hypothyroidism.
• Any condition, which, in the opinion of the investigator, may interfere with the participation of the subject in the study (e.g. difficulty to meet the requirements of the study, attend the consultations, answer the questionnaires).
• Report of significant clinical problems by the subject's partner, such as decreased sexual interest, pain during relations or other forms of sexual dysfunction that interfere in the relationship with the sexual partner.
• Patient or partner willing to undergo a surgical procedure during the study period, which may interfere in the results.
• Peyronie's disease, which may lead to impossibility to penetrate or cause pain during the sexual intercourse.
• Use of nitrates.
• Illiterate patient.