The answer to the question "what happens if poison expires?" is that it depends on the type of poison. Some poisons may become more toxic, some may become less toxic, and some may become differently toxic, so that they are no longer as effective against their target but could cause severe harm to other organisms. For example, some organic poisons such as common herbicides and insecticides tend to break down, meaning a larger dose is required. They will eventually lose their toxicity but it will take a very long time. Some organic poisons, however, break down into even more toxic substances. These usually have a very short expiration date. Inorganic poisons (such as arsenic) only have expiration dates because they can be contaminated by water vapor which changes their chemistry. This means that they generally take longer to work, so rather than killing you in a couple of hours it could be a couple of months before you die. Ironically, the altered version of arsenic is actually more toxic than the normal version. It just takes longer to work so its less useful. It is important to note that poison is just a chemical, and time affects it in the same way other chemicals change over time. Expiration dates are just estimated shelf life of a particular item, and they should not be strictly followed.