I got my hands on some expired creatine how effective would it be?
Well that depends on how it was stored. As long as it was stored in proper conditions(not in extreme heat or cold) or was subject to anything to denature it, I would assume that it should be at least 80% effective, as long as it has expired fairly recently.
Buff is correct. If you open it and it’s all clumped together I would toss it (means it has been exposed to air and moisture). If it looks good though and isn’t more than a year past the expiry date I’d go ahead and use it.