Smart drinks are magical alchohalic beverages which causes the drinker to become temporarily highly intelligent and allows them to think quicker and clearer. The hangover produced by smart drinks are as strong as the difference between the drinker's reqular intelligence and the IQ produced by the drinks. That's why they're severly dangerous when taken by blithering idiots. Even when drunk by people of average intelligence they produce, as Ferd put it, "the granddaddy of all hangovers." Smart drinks drunk around the same time as beenuts cause the consumer to have horrible gas, bad enough to make people choke and run screaming. It's like the old rhyme goes:
Smart drinks then beer, never fear
Beenuts, then smart drinks, nasty fart stink.
The night before the in vasion of Fantastic! Ferd had unwittingly both eaten beenuts and drunk smart drinks. That accounts for the horrible gas he had the following day in bagel form. This gas, however saved Ferd's life as well as Jorge's, Barry's, and Lon's as it kept anyone from eating them.