Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil. Percentage of Patients Reporting an Improvement in Erections. At the same time, on-treatment function was better in treated patients who were less impaired at baseline. Based upon measurements of sildenafil in semen of healthy volunteers 90 minutes after dosing, less than 0. This will increase the supply of blood to the lungs and reduce the workload of the heart. Maximum observed plasma concentrations are reached within 30 to minutes median 60 minutes of oral dosing in the fasted state. Your doctor may need to change the doses of your medications or monitor you carefully for side effects.VIAGRA does not protect against sexually transmitted diseases, including HIV. The most common side effects of VIAGRA: headache; flushing; upset stomach; This Savings Offer is limited to 50% savings off the amount of your co-pay or your out-of-pocket cost, whichever is less, with a maximum savings of $ off each prescription for up to The generic version of Viagra, sildenafil works in the same way for a much lower price per dose. It comes in 25, 50, and mg options starting at $4 per dose. Hims also recommends sildenafil for PE. Key facts. 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.