Take sildenafil as needed before sexual activity. 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. If you are a Mayo Clinic patient, this could include protected health information. Viagra is always taken as needed, usually an hour before having sex. It can be dangerous to take sildenafil if you also take medicines called nitrates often given for chest pain. Stop taking VIAGRA and call your healthcare provider right away if you have any sudden vision loss sudden hearing decrease or hearing loss. What special precautions should I follow?7 Over The Counter Medications for ED. Depending on the cause of erectile dysfunction, the most effective treatment can vary. Typically, the best way to treat erectile dysfunction . Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). In combination with sexual stimulation, sildenafil works by increasing blood flow to the penis to. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 .