These include itraconazole Sporanox, Tolsura , ketoconazole Extina, Ketozole, others and ritonavir Norvir. The mean steady state volume of distribution Vss for sildenafil is L, indicating distribution into the tissues. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure. Use only the brand of this medicine that your doctor prescribed.That’s compared with more than $50 per pill for Viagra. And while generic Revatio is available only in a mg strength—Viagra comes in , , and mg tabs—not to worry: Your doctor can. Viagra dosage for erectile dysfunction (ED) is usually milligrams (mg), but those over 65 may start at 25 mg. Your doctor will also need to consider possible drug interactions. Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). In combination with sexual stimulation, sildenafil works by increasing blood flow to the penis to.