I don't need to google it. Medications have an expiration date because that is how long the company guarantees them to be effective by a certain percentage. Say, 98% effective for 2 years, after that the efficacy decreases with each year that goes by until you might as well just pour water on them. Same goes for a rabies vaccine for your pet, after 1yr. or 3 yrs. the titer level starts to drop, they still have some protection from it, but I wouldn't want to bet their life that it was enough. LOL.