Without taking shit-loads of ketamine the easiest way to put up that wall of obfuscation is by forcing a player to modify their speech pattern. It would be too easy to let them choose their own gibberish, plus theres a good opportunity to make this a fun distraction.
Buy/Steal/Make some children phonetic 'sound shape' cards like the image below
Then whenever your players finds themselves ingesting a quantity of drugs (or possibly other consumables) have them make a constitution save, or save v poison or somesuch. If they pass they're fine, if not deal them 9 cards - they can only say (I mean in the very literal sense, not just character talk) whats on those cards until they pass a constitution check, they can make one a turn. If they break the rules, they'll lose a HP for every infraction.
Of course every good drug/poison has an antidote or an upper to the downer, so should your drugs - make available in the world a solution, as whilst it might be amusing for a turn or two - I imagine players getting pissed off if this is used too frequently.