If you ever need emergency medical treatment for a heart problem, the healthcare providers who treat you will need to know when you last took sildenafil. If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food. Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. Follow the manufacturer’s directions to use and clean the oral syringe. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly.In most males,* Viagra works within 1 hour after it’s taken. It’s possible that Viagra can begin working within 30 minutes of being taken. But other times, it may take up to 4 hours to start.  · Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase 5 (PDE5) inhibitors. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly. The penis is one of the areas where this enzyme works. RX Home Delivery. The Costco Member Prescription Program is NOT insurance. It is a prescription drug discount card program that provides eligible Costco members and their eligible dependents with the ability to obtain lower prices at participating pharmacies. Any Costco Member can join. Persons under 18 or pets are eligible as a dependent.