From the God that is Barry Cryer:-
Chap gets up from his ward bed and walks out of hospital in only his gown and pulling along his drip on its tripod wheels, and goes into the pub opposite. As he enters the crowded pub, the people part to let him get up to the bar and says quietly to the barman
"I'll have a double whisky please"
Barman gives it to him, which he knocks back in one go and he leans forward to whisper.
"Of course, I shouldn't be having this with what I've got"
"What have you got?" says the concerned barman