Antibiotics, antifungals, and HIV protease inhibitor drugs are all processed in the liver, just like Viagra. Viagra can help with this situation or in others if you need it. If you already have a primary care doctor, they would even know your medical history and current medications. How oral medicines differ Each oral medicine for ED has a slightly different chemical makeup. It lasts 4 to 5 hours, or longer if you have mild to moderate ED.Don’t be fooled into buying “herbal viagra.” Never take any medicines that claim to be the “herbal” or nonprescription version of an oral medicine for ED. These aren’t an . I mean who is he to say it shouldn’t be used. It IS a legal drug. Every doctor has to use his professional opinion for EVERY prescription that he gives and each patient is different. . Do not use more Viagra than your doctor prescribes. Using more than this can lead to serious side effects. What to do in case you take too much Viagra.