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As we all know that,

Any poison is nothing but a chemical compound.

And as discussed in the question: Chemicals-do-have-an-expiry-date!

So, my question is: Is poison still poisonous after its 'expiration date'? and Is every poison always as poisonous as just after it was synthesised ?

please feel free to update with the suitable tag(s).

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    $\begingroup$ The problem here is tagging compounds with "expiration date" often is very unscientific. $\endgroup$ – M.A.R. Apr 19 '16 at 19:04
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    $\begingroup$ You see, there is no physical phenomenon as "expiry date". Unless something happens to the toxin, i.e. it degrades, reacts with moisture, air etc. that eventually kicks in, there's no clock ticking counting towards the 'expiry date'. $\endgroup$ – M.A.R. Apr 19 '16 at 19:17
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    $\begingroup$ Commercial chemical products carry an expiration date for legal reasons: It's the date past which the company no longer guarantees the product to work properly, remain safe, etc. $\endgroup$ – hBy2Py Apr 19 '16 at 19:21
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    $\begingroup$ An expiration date is merely a guarantee that the product will still have a certain potency when stored as recommended given known ways it can degrade. It doesn't suddenly change at the date: it is just a statistical average. So, for things that do degrade (not all poisons will, like elemental lead), the date simply means the poison might be slightly less effective that it was before the date not that its poisonous effect will suddenly disappear. $\endgroup$ – matt_black Apr 19 '16 at 21:05
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    $\begingroup$ Protip: Poisons are not the opposite of food. Just because you can eat food before it's expiration date doesn't mean it's a good idea to eat poison after it's expiration date $\endgroup$ – PyRulez Apr 20 '16 at 4:56
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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.

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    $\begingroup$ @ABcDexter: The existing example (cinnabar mineral) wasn't synthesized at all ! You assume poisons are synthetic, when you say "just after it was synthesised". But many are naturally occurring. $\endgroup$ – MSalters Apr 19 '16 at 21:20
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    $\begingroup$ So many see the word "chemical" and get the heebie jeebies. Few folks stop to consider that if you're allergic to strawberries they can be just as deadly as cyanide. What is a strawberry? Just a sack of chemicals. $\endgroup$ – MaxW Apr 20 '16 at 1:08
  • $\begingroup$ Yes, agreed. But again, 'some' naturally occurring chemicals like strawberries do have an expiry date after which they rot, right? $\endgroup$ – ABcDexter Apr 20 '16 at 4:31
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    $\begingroup$ @ABcDexter: No, they pretty much start to rot as soon as they're harvested. (But that's biology more than chemistry). $\endgroup$ – MSalters Apr 20 '16 at 10:43
  • $\begingroup$ Ok, so that comes under botany or boichemistry :-) $\endgroup$ – ABcDexter Apr 20 '16 at 18:03
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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).

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"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.

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