All PDE5 inhibitors, including Viagra and Cialis, require a valid prescription regardless of the pharmacy or provider. In a study of healthy male volunteers, co-administration of sildenafil at steady state 80 mg t. The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. Make sure you have discussed any other medications you are currently taking or plan to take before starting sildenafil with your doctor and pharmacist. Erectile dysfunction medications require a prescription from a licensed provider. Do not stop taking sildenafil without talking to your doctor.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. Sildenafil Citrate Coupons, Prices, and Savings Card. Brand Names: Revatio, Viagra. Sildenafil is used to treat male sexual function problems (impotence or erectile . Headache, flushing, and upset stomach are common Viagra side effects. These effects are usually mild and often resolve on their own. While rare, Viagra can cause an erection that lasts longer than 4 hours (priapism). This can result in permanent damage if left untreated. Vision changes are also possible.