Sildenafil should not be used with any other medicine or device that causes erections. However, if it is almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Use only the brand of this medicine that your doctor prescribed. Do not use this medicine if you are also using a nitrate medicine, often used to treat angina or high blood pressure. Medicines that you take by mouth are called oral medicines. Do not mix the liquid with other medications or add anything to flavor the medication. Use the oral syringe provided with your medication to measure and take your dose.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. This boosts blood flow and helps you get an erection from sexual activity. How oral medicines differ. The recommended dosage of Viagra for erectile dysfunction (ED) is 50 mg, taken as needed about 1 hour before sexual activity. You can take it 30 minutes to 4 hours before sexual activity. And you. You can take the mg and mg doses as shortly as 15 minutes before sexual activity. As a second-generation medication, Stendra also seems to have milder side effects than Viagra and the.