Its effects often last for 4 to 5 hours. This medicine has been approved in the United States to treat premenopausal females who have low sexual desire. If you are taking sildenafil for erectile dysfunction, you are unlikely to miss a dose since this medication is taken as needed, not on a regular dosing schedule. Follow the manufacturer’s directions to use and clean the oral syringe. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food. If you have an erection that lasts more than 4 hours, get medical help right away. These medicines are called PDE5 inhibitors.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. Valid at all major chains including Walgreens, CVS Pharmacy, Target, WalMart Pharmacy, Duane Reade and 65, pharmacies nationwide. No manufacturer promotions could be . CVS – Online Drugstore, Pharmacy, Prescriptions & Health Information. Refill and transfer prescriptions online or find a CVS Pharmacy near you. Shop online, see ExtraCare .