All drugs and vitamins start to age or break down the day after they are manufactured. Most all products are safe to take up to 1 year after expiration date. The concern is that the product may be less effective, the dosage may vary. Not all substances deteriorate at the same speed. For example: Vitamin A deteriorates one third faster then other vitamins. Quality vitamin manufacturers will beef up their vitamin A slightly to cover the time period that this vitamin may be on their shelf, distributors shelf, retailers shelf or in your home. A quality manufacturer will be sure their vitamins are approximately at the listed strength on the label at time of expiration.
In 1937 over 100 people died after ingesting " Elixir Sulfanilamide " manufactured by . Massengill Company of Tennessee. The product was formulated in diethylene glycol , a highly toxic solvent that is now widely used as antifreeze.  Under the laws extant at that time, prosecution of the manufacturer was possible only under the technicality that the product had been called an "elixir", which literally implied a solution in ethanol. In response to this episode, the . Congress passed the Federal Food, Drug, and Cosmetic Act of 1938 , which for the first time required pre-market demonstration of safety before a drug could be sold, and explicitly prohibited false therapeutic claims.