Some types of erectile dysfunction medication (such as Viagra, Levitra, and Cialis) are taken orally. Known as phosphodiesterase (PDE) inhibitors, these drugs are taken one hour prior to sexual activity. While oral forms of erectile dysfunction medication improve the response to sexual stimulation, they do not trigger an automatic erection as injections do. Drugs like papaverine hydrochloride, phentolamine, and alprostadil are injected into the penis -- and alprostadil can alternately be inserted into the urethra in pellet form, through a system called MUSE.