it depends on what condition is affecting your cat, too -- there are two formulations of B12 -- cyanocobalamin and methylcobalamin
most human-intended B12 is cyanocobalamin, as is most of the B12 in a B complex pill -- I'm looking at the super B complex pills from Costco, and those have cyanocobalamin
each of those B12 formulations targets a different feline condition
would be a bit leery of supplying a cat with a B complex -- there are a LOT of ingredients in the one I'm looking at, and I am skeptical about the safety of some of them for felines, especially since you would be breaking up a pill or capsule and there is no guarantee that the particular vitamin that's your target, would be distributed evenly within the pill or capsule
sodium chloride? nope, cat gets enough salt as it is; cellulose? cat can't digest that, there are better fiber sources; potassium chloride? suppose it balances the sodium, but cat gets enough in regular "balanced" food; silicon dioxide? possibly a fancy name for "sand"; carnauba wax? what on earth does a cat need that for, indigestible, usually used to wax CARS not cats ; mannitol?? a sugar alcohol, you do NOT want to give a diabetic cat this !!!!!