Expiration dates are maybe a hedge by the manufacturer? Our pediatrician said of some medication we had on hand that it’s probably good way past the printed dat

Expiration dates are maybe a hedge by the manufacturer? Our pediatrician said of some medication we had on hand that it’s probably good way past the printed date, maybe a bit less effective over time, and I’ve heard this about Aspirin, for example. What drugs actually break down quickly over time of kept in the recommended temperature range?