Chemistry - Is poison still poisonous after its 'expiration date'?

Solution 1:

It depends on what the poison is. If we take the colloquial use of the word and include toxins and venoms, many are things like proteins that will certainly denature or otherwise degrade, eventually becoming harmless. e.g. tetrodotoxin, ricin, botulinum, etc. I would expect that type of poison to have the shortest shelf-life as they are relatively fragile.

Many other poisons are small organic molecules. These can often be degraded by oxidation in air, exposure to UV, hydrolysis etc. and would include things like nicotine and nerve agents like sarin and VX. Many nerve agents, have shelf lives of a few years and research has actually been done to extend them for use in munitions.

Several metals are known to be poisonous (like lead, mercury, and cadmium) and are problematic because they are toxic in not only their elemental forms, but also in inorganic and organic compounds. There may be a great difference in toxicity of the different forms, (see elemental mercury vs methylmercury), but most forms remain at least somewhat toxic. These may last a very long time because reactions likely to occur under normal conditions may not render them safe, e.g. a chunk of cinnabar ($\ce{HgS}$ mineral) sitting on your desk will not undergo any significant change to render it safe, even on a geological timescale.

Solution 2:

Chemicals can also decompose due to contained impurities. I would generally and universally agree that deterioration does occur over time. The extent of which varies. I would also say that business reasons, aside from legal reasons, also play into the expiration date. See also concepts of shelf-life and pot-life.

Also, the decomposition or reaction product(s) may or may not be relatively benign compared to the reactant(s).

Solution 3:

"Not up to quality" for a poison could mean anything. It could be more toxic; it could be less. It could be differently toxic, so that it's no longer as effective against its target but could cause severe harm to other species. It only means that one chemical may have started to degrade, and the end product of that degradation could be very different. 

So don't get too hung up on the category "poison". Poison are just chemicals, and time affects them in the same way other chemicals change over time. And don't get too hung up on expiration dates, either: they're just guesses given the wide range of storage conditions.