system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why vitamins have expiration dates. Do vitamins and minerals degrade?
gpt
Many chemicals oxidize and lose their intended properties, just like food (which incidentally is made of chemicals :P). This means that free radicals (oxygen ions) attach to different parts of the vitamins/rip off a few atoms.

So the vitamins could either become inert and useless, or potentially poisonous.

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More ELI5:

Our air will over time cause the vitamins to change their chemical structure. This makes vitamin A no longer vitamin A - but some deviant variation.