system
Given the following post, respond as if you're commenting on Reddit:
human
How are expiration dates assigned to medicine?

Most times the components in medicine (either pills or syrups) are chemicals. What do you take into account to set an expiration date?
gpt
Often drugs will naturally degrade over time, as the chemicals are relatively stable, but not completely so (or exposure to air, water, or temperature changes will degrade them). Expiration dates are typically set after a period of time when it is believed the dose will vary significantly from what is on the label under standard conditions.