It works by blocking the phosphodiesterase enzyme, which normally breaks down a substance known as cyclic guanosine monophosphate cGMP. The maximum recommended dosing frequency is once a day. Expand section Collapse section What is Viagra? Stress can cause various illnesses. The main measure of effectiveness was based on the ability of the men to get and maintain an erection. Why has Viagra been approved? Patients with liver problems or severe kidney problems should start treatment with the 25 mg dose.Assessment history Authorised This medicine is authorised for use in the European Union. Overview This is a summary of the European public assessment report (EPAR) for Viagra. By Michelle Roberts Health editor, BBC News online Men will no longer require a prescription to obtain the impotence drug Viagra and will instead be able to buy it over . Still, if your doctor says it’s okay for you to use and you end up paying out-of-pocket, you can still get the generic for a lot less than Viagra. According to price-comparison site .