Depends on whether you are drinking it yourself or feeding it to someone. Especially if that someone surviving is really bad for you.
Can poison even get expired? And who cares? just put in enough to kill then throw in another 10 times the previous amount. If the guy getting poisoned survives, you'll know he's an MC with thick plot armor and you're about to die.
First off, "expiration dates" are not some kind of deadline, where it was fine yesterday and worthless tomorrow. They're the time in which the manufacturer claims that the product will be up to their quality standards. It makes no claims after that. "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.
poison obviously, is't a chemical and any chemical can become useless, oh maybe it depends on how poisonous the poison is
If were talking people being poisonous in the forums, then I am... the hidden poison within the concocted jewel-like beverages of grand-delusions~ Drink me, eat me~ you know want to~ let's dance in this hell together~ forever and ever~ dead men~