"The bartender grinned and waved a young man over to the bar. / 'Joey,' he instructed, 'get me some tabasco sauce, cayenne pepper, chili peppers and mustard seed.' / The young fellow nodded and ran into the back room, as the bartender reached behind the polished wooden shelf and pulled out a bottle of vinegar and part of an onion. His helper rushed back with an armful of jars and bottles and dumped them on the bar. The bartender whistled tunelessly as he mixed all of it in a beer glass, then glanced up at Marty. / 'In ten minutes,' the barkeep remarked cheerfully, 'he'll be as sober as a priest on Sunday.' "
"The bartender leaned over the bar, handing Marty both the glass, which was now filled with a uniform, vile brown liquid, and a clothespin. / 'Put the clothespin over his nose,' the bartender instructed. 'When he opens his mouth, pour it down his gullet. Then stand back.' / Marty did as he was told, placing the clothespin over the bridge of Doc's nose, effectively closing off his nasal passages. The sleeping Doc opened his mouth to gulp in air. In a single fluid motion, Marty poured the contents of the glass into Doc's gullet."
It caused Doc to immediately wake up, run outside, and dunk his head in the horsetrough (which Chester had earlier told "Clint Eastwood" was the only place to get a drink of water at the saloon).
Chester had to administer wake-up juice to Doc when he took a drink on July 4 and again on September 7.
Behind the scenes
According to the novelization (see first Quote above), the wake-up juice contained green pepper juice,tabasco sauce, cayenne pepper, chili peppers, onions, and mustard seed. However, in the movie, only five ingredients are mixed together and no chili peppers are added to the drink.
In commentary to the DVD of Part II, Bob Gale noted that things like "wake-up juice" might have been something that bartenders knew how to make in 1885, but that the knowledge of that and other homemade mixtures had been lost to history, long forgotten a century later. It was also possible that the recipe included stimulants that were still legal in 1885, but which are prohibited now — including cocaine.