Don't ask how I know this: Nitroglycerin tablets come in a tiny, sealed glass jar. It's a weird little container. So I wonder if it's short-lived in air / humidity. If so, then replacing it periodically might be the prudent thing to do.

Even at my workplace, which is a much more tame environment, somebody goes around and checks the contents of the first aid kits that are located around the buildings, periodically.