Vitamins past their expiration date lose potency and may not be as effective to treat vitamin deficiency or to provide extra nutrients! purchase vitamins that contain a printed sale by date or expiration date.
Most everything lasts a while past expiration before deterioration, including food. Expiration dates have a margin of safety built in. That being said, the above is true, but vitamins usually have way more that anyone needs in them so a little deterioration in potency isn't that big a deal depending on why you're taking them. After all, much of them just goes down the toilet anyway, so again, a margin of safety is built in there, too.