Read and follow these instructions carefully before you start using sildenafil and each time you get a refill of your medicine. Store mixed oral liquid in the refrigerator. As with sildenafil, a high-fat meal or alcohol keep the body from absorbing it as quickly. It works for up to 36 hours. If your dose is different, do not change it unless your doctor tells you to do so. Its effects might last longer if you have mild to moderate ED. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles . Mild side effects of Viagra can include:*. headache. flushing. indigestion. mild and temporary vision changes, such as a blue tinge in your vision, blurred vision, and sensitivity to light. . Read and follow these instructions carefully before you start using sildenafil and each time you get a refill of your medicine. Ask your doctor if you have any questions. Adults 65 .