Tadalafil Cialis. However, if you’d feel more comfortable talking with a healthcare provider virtually rather than in person, you can consider requesting an online ED prescription instead. Vardenafil usually works for 4 to 5 hours. Do not forget that there are different companies that are providing this medicine and each one of them has its own version. You need a prescription from a doctor to obtain sildenafil. Viagra generally only stays in your system for four to six hours. Your healthcare provider can help you determine the right dosage for you.Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work. Get the facts on five over-the-counter ED pills. Learn how options like DHEA and yohimbe can help treat erectile dysfunction. Learn about warnings and more. Tell your healthcare provider about all the medicines you take, including prescription and over-the-counter medicines, vitamins, and herbal supplements. VIAGRA may affect the way other medicines work, and other medicines may affect the way VIAGRA works, causing side effects. Especially tell your healthcare provider if you take any of the following.