Not sure whether you are asking about taking it out of the country or bringing it back in?
I don't think I have ever been asked for proof of prescription for anything leaving the country. I was once pulled for a baggage inspection in Australia and I was asked about some blood pressure tablets I had, I didn't have the prescription with me and all they did was a swab test and let me through with them.
If you are thinking about bringing viagra back into the country I'd ask you why ? You can buy viagra online easily & cheaply here or even over the counter with a brief q&a with the pharmacist.