If it was fully intrusive, as in the snake camera all the way up to your nostrils, they'd have given you the "clean oot" delicious* drink that vacates the tubes for you. You've no' been strapped tae a lavvy for 24 hours in doing so, therefore it is hopefully just a wee routine "peripheral" check up. Plenty Listerine though, mate, although if the doctor's muddling aboot yer erse, i'm not sure a bit of drink-induced halitosis is going to be readily noticed.
Hope things go well buddy