Vitamin's past expiration: less beneficial? Not at all beneficial? Harmful?

Are multi-vitamins that are past their expiration date only less beneficial? Or not beneficial at all? Or do they become actively harmful?

Of course, I cannot imagine that suddenly on the date on the bottle it goes from good to completely useless, so I suspect “not beneficial at all” only applies to very old vitamins.

I’ve often wondered the same thing myself:

Doesn’t give a very satisfying answer though.
Between you and me, I think I’d stick to the expiration dates.

If you keep them in the refrigerator, you probably won’t have to worry about it.
If they smell different than when they were new, you should probably throw them out.

Pharmacists will often tell you, sotto voce, that solid pills are stable for years after the expiration date – suspensions and liquids far less so. Nevertheless, no one will guarantee it, myself included, especially professioanlly. Naturally, you use them at your own risk.

Mostly, they lose a bit of potency. But if they smell funny, or if the expiration date is WAY past, or if they look odd (like the coating has lost it’s protective abilities) then toss 'em.