I am a nurse and I have taken a lot of classes in pharmacology. Anyway, expiration dates are just that. This is the date that you are to use by to get maximum benefit. As the date goes past the expiration date, the drug degrades. This causes less of the active drug to be available. If you take an asprin that is past the date, then you are getting less of the affects you would get if you took one with a valid date. Basically, the vitamins can possibly be weaker. The further way from the date, the more weaker they can be. Notice I stated "can possibly be".... there is no way to say for sure.

Professionally, I will state that you should never take them past the expiration date. However, personally, I know a lot of people that will increase the amount such as taking two vitamin pills a day if you are taking old pills. It is your choice.