This type is best if you do not plan to have sex regularly, but want to have the medication on hand. Some examples of nitrates are nitroglycerin Nitro-Dur, Nitrostat, others , isosorbide mononitrate Monoket and isosorbide dinitrate Bidil. These PDE5 inhibitors may cause headache, flushing, drop in blood pressure, or other adverse effects. Our guide to maintaining an erection naturally shares lifestyle tips that you can use with Viagra or Cialis for improved erections and better health. Instead, both Viagra and generic sildenafil are only available with a prescription. Erectile dysfunction medications require a prescription from a licensed provider. Oral medicines for ED usually work well and cause few side effects.Dosage range: mg to 5 mg. Frequency: once daily, regardless of the timing of sexual activity. Cialis’s recommended dosage when it’s taken as needed for ED is as follows. Dosage range: 5 mg to 20 mg. Frequency: one dose at least 30 minutes before sexual activity, with a maximum of one dose per day*. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. Both Cialis and Levitra are FDA-approved to treat: Erectile dysfunction (ED) in adult males.* ED is the inability to achieve or sustain an erection firm enough for sexual activity.