Do not take more or less of it or take it more often than prescribed by your doctor. Antibiotics, antifungals, and HIV protease inhibitor drugs are all processed in the liver, just like Viagra. Brand names Why is this medication prescribed? This increased blood flow can cause an erection. You can take it in a small dose daily or in a larger dose as needed. The four main medicines taken by mouth for erectile dysfunction are: Avanafil Stendra. Severe liver disease.Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 . Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or alcohol. Its effects . VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are .