In your circumstances, they should be able to make exceptions.
My mum was on water tablets and had a tiny bladder, to boot, poor thing!
It was a family joke that she kept a notebook, giving star ratings to all the toilets she had to visit.
I'd have a word with your GP, to see if they could mention this to your dentist.
My mum would make sure she 'went' before leaving the house, but even in the time it took for the journey to the dentists, plus the length of time for the consultation, she may well have needed to go again. Everyone's different!