1. Katzenstein L, Grossman EB: Viagra (sildenafil citrate): the remarkable story of the discovery and the launch. New York: Medical Information Press; 2001: 90.
2. Basar M, Tekdogan UY, Yilmaz E, et al.: The efficacy of sildenafil in different etiologies of erectile dysfunction. Int Urol Nephrol 2001, 32: 401–407.
3. The Process of Care Consensus Panel: The process of care model for evaluation and treatment of erectile dysfunction. Intl Impot Res 1999, 11: 59–74.
4. Stekel W: Impotence in the Male: The Psychic Disorders of Sexual Function in the Male, vol one. New York: Boni and Liveright; 1927: 329.
5. Masters WH, Johnson V: Human Sexual Inadequacy. Boston: Little, Brown; 1970.