The products discussed in this site may have different product labeling in different countries. Read our editorial process. An erection results from an increase in blood flow to the penis. All PDE5 inhibitors, including Viagra and Cialis, require a valid prescription regardless of the pharmacy or provider. Some platforms have a digital pharmacy that ships the medication to your home, while others send an electronic prescription to your local pharmacy. Do not take a double dose to make up for a missed one.CHOOSE BRAND-NAME VIAGRA. The original little blue pill prescribed by doctors for over 20 years. Clinically proven to treat men with erectile dysfunction (ED) Ongoing . Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise .