I think creatinine is an essential dietary requirement for cats anyway. I need to go check, but I think it's one of those things they put in "food" cat food, as opposed to cast treats, which aren't nutritionally complete.
Probably not good for thier kidneys? As creatinine levels are tested to indicate kidney health. There also could be other things in human creatine supplements that would be bad for cats, such as chocolate flavouring.
Yeah. I finally went and looked it up. I can't find anything that says it's bad for cats, but high creatinine levels is an indication of kidney disease - so it's not necessarily a cause, but it's an effect and so probably best avoided.
I was misremembering; the list of supplements cats require in their (processed) foods for them to be nutritionally complete is long.