What happens when a poison expires?
The effects of poison after validation date.
Many organic poisons like most common insecticides and herbicides break down over time and eventually lose their toxicity. They become less poisonous, meaning you need a bigger dose, but you’d have to wait years after the expiration before it stopped working altogether.
Some organic poisons have an expiry date for exactly the opposite reason. They break down, but they break down into more poisonous substances. Some herbicides for example gradually decay and produce dioxides, some pesticides break down when exposed to water and produce a very potent volatile nerve gas. You can usually spot these poisons because the expiration date is very short and is printed as two dates: expiration date and shelf life from date of opening.
The inorganic poisons like arsenic or mercury compounds tend not to break down as such. They have expiration dates because they absorb moisture from the air and change form chemically. That makes then harder to use and it also means they take longer to work. Instead of killing the animal within hours it might take days or months.
Ironically the chemically altered form of arsenic is often more deadly insofar as it requires a smaller dose to be lethal. It’s just less useful because it takes a long time to act.
Although it depends on the poison! Some denature (if they’re proteins) or decompose, which would change them into other chemicals. In that instance, it would no longer be possible to say how much of which chemical was in the bottle. For instance, hydrogen peroxide decomposes over time, producing water molecules and oxygen gas, which significantly changes its chemistry (and makes it less dangerous to humans).
Venoms, which are proteins, will denature over time and will become useless and stop working. You can’t keep snake venom in a jar forever, and that’s why snake venom for making antivenin is refrigerated during transport – it slows down denaturing.
Other chemicals become MORE poisonous over time, but I don’t know of any examples offhand. Certainly some explosives, such as dynamite (which is a mixture, not a compound) will separate and become very dangerous if given enough time.
So the answer is, it can become less poisonous, it can become more poisonous, but poison is always poison.