Erectile dysfunction was defined by a National Institutes of Health conference in 1993 as 'the inability to achieve or maintain an erection sufficient for satisfactory sexual performance (for both partners)' (National Institutes of Health, 1993). The degree of erectile dysfunction may be graded according to the number of satisfactory attempts out of 10 (mild 7-8, moderate 4-6, and severe 0-3) (Albaugh and Lewis, 1999).