Vitamins gradually lose potency as the pills become exposed to sunlight, oxygen and moisture. While they may not become toxic or harmful, expired vitamins may not provide the results you want. Vitamin manufacturers set expiration dates by performing stability tests. These tests show how much the product will degrade over time. Vitamin manufacturers then add enough of the raw nutrient to compensate for any lost potency. Furthermore, certain vitamin formulations contain fillers and stabilizers that have a recommended shelf life.