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.

Taste it and tell us your observation later.