It assumes you haven’t any other underlying problems. A lot of people have.
I went to my UK GP (doctor) and told him I had suspected ED. He prescribed 50 mg but told me most people need 100 mg. I was told to return after I had used the 50 mg tablets. Without batting an eyelid he changed the prescription to 100 mg.
During the consultation we discussed my heart attack, the stent that had been fitted, and the blood pressure medications I was on. Also my diabetes. He was still happy to prescribe Viagra. He did mention to keep off poppers.
So the answer Is to take professional medical advice.
Incidentally they are free in the UK in certain circumstances, so I save them up to use in Thailand! Get 4 per month, which does me for my 3 or 4month visits.