He spoke about drinking on duty, though - even during working hours and crises. See "Tholian Web" where he thinks the cure for the week's space-sickness would go down better with Scotch. Although the cure itself was supposed to be an inebriant of sorts, so...
It was a Klingon nerve gas...
SCOTT: What is it?
MCCOY: It's a diluted theragen derivative.
SPOCK: Theragen? A nerve gas used by the Klingons.
SCOTT: Aye. And deadly, too. What are you thinking of, Doc? Are you trying to kill us all?
SPOCK: If I remember correctly, it caused fatality only when used in pure form.
MCCOY: That's right, and in this derivative, mixed with alcohol, it merely deadens certain nerve inputs to the brain.
SCOTT: Oh, well, any decent brand of Scotch will do that.
MCCOY: One good slug of this, and you can hit a man with phaser stun, and he'd never feel it or even know it.
SCOTT: Does it make a good mix with Scotch?
MCCOY: It should.
SCOTT: I'll let you know. (leaves with the remaining contents of the flask)
I'm pretty sure Scott was the one with the problem.
Hardly, Scotty's just observing that the side effects of the Theragen mix would have the same sensation as a good Scotch (Coal Ila, Dalwhinnie, Bruichladdich, Lagavulin, etc.) which does deaden nerve impulses to the brain (you feel invincible and charming), and since McCoy mentions that it is to be mixed with alcohol Scotty would merely prefer to try it with his own favourite type, as opposed to the American swill McCoy would've mixed it with. Personally the only problem I see with this exchange is that Scotty wants to mix something with Scotch (a dram of water, on the rocks or straight, no Klingon poisons please).