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 or high blood pressure), hormonal abnormalities (such as low testosterone levels), and use of some drugs (such as some treatments for high blood pressure or depression). Phosphodiesterase-5 inhibitors are drugs available to treat ED. Commonly prescribed ED drugs include sildenafil (known by the brand name Viagra [Pfizer, New York, New York]), vardenafil (known by the brand name Levitra [GlaxoSmithKline, Brentford, United Kingdom]), and tadalafil (known by the brand name Cialis [Lilly, Indianapolis, Indiana]). Because up to 40% of middle-aged and older men have some degree of ED, drugs for this condition are commonly used. Risks for sexually transmitted diseases (STDs) may accompany the increased sexual activity that these drugs enable. Sexually transmitted diseases include infections with viruses or bacteria, such as HIV, chlamydia, gonorrhea, herpes, or syphilis. Little is known about the risk for STDs in men who use ED drugs.