Other than experiencing an off-tasting beer or one that has lost some carbonation, there are no after-effects from drinking expired beer. Beer with expiration dates or, in some cases, born-on dates, is a recent phenomenon made popular by the Anheuser-Busch brewing company.
Because of the fermenting and bottling process for beer, which forces oxygen inside each bottle, it is thought that beer begins to lose its flavor right after the bottle cap is secured or the can or keg is sealed shut. In general, it is believed that beer is at its best until 6 months have passed after bottling. Then, the carbonation begins to deplete and the flavors begin change, often resulting in what some term to be a "skunky" beer. This occurs because oxidation affects the taste of the beer and the product's body, or flavor, begins to weaken or fade.
In order to keep beer fresher and better tasting longer, it should be stored in a cool dry, dark place, especially beer in clear or light-colored bottles. The temperature should remain stable as warming and cooling beer repeatedly will cause it to break down sooner.
Many beers now have an expiration date somewhere on the can, bottle or packaging, which can vary by manufacturer. Anheuser-Busch, maker of Budweiser, Bud Light and Busch beer, was at the forefront of this dating process, debuting "born-on dates" on its products in 1996.