Erectile dysfunction (ED), or impotence, is an inability to get or keep an erection firm enough for sexual intercourse. Causes can be psychological or physical. Physical causes include advanced age, heavy drinking, chronic diseases (such as diabetes), hormonal abnormalities (such as low testosterone levels), and use of some drugs (such as some treatments for high blood pressure or depression). Because hormonal abnormalities can cause ED, some doctors will routinely measure hormone levels in men with this condition. However, the value of this testing in men who have no other signs of hormone problems is unclear. Phosphodiesterase-5 (PDE-5) inhibitors are drugs available to treat ED. Commonly prescribed PDE-5 inhibitors include sildenafil (known by the brand name Viagra [manufactured by Pfizer, New York, New York]), vardenafil (known by the brand name Levitra [manufactured by GlaxoSmithKline, Brentford, United Kingdom]), and tadalafil (known by the brand name Cialis [manufactured by Lilly, Indianapolis, Indiana]).