It has come to my attention that a very popular flavoring agent thought to strictly vegetarian is not. When purchasing what is called "nutritional yeast" check to make sure of the ingredients. This may be hard to run down, but do not take it for granted that it is not contaminated with animal products. Many times the product is fortified with B-12 derived from an animal source.
Rather than purchasing a product when there is doubt, look for brewers yeast that states there are no additives.