Click here for an email preview. A newer form of the medicine that dissolves on the tongue might work faster than the pill that you swallow. 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. If you are taking sildenafil to treat PAH, you will probably take sildenafil three times a day with or without food. Tell your doctor if sildenafil is not working well or if you are experiencing side effects. Different brands may not work the same way.Overdose Ask your doctor Q&A Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a .  · For oral dosage forms (suspension or tablets): Adults—20 milligrams (mg) 3 times a day. Your doctor may adjust your dose as needed and tolerated. . The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called .