An expiry date is not an exact moment when a medicine becomes dangerous, but it is the last date the maker can promise full potency and quality under proper storage. After this point, the medicine can slowly lose strength, which matters most for treatments you rely on to work fully.
Storage affects this strongly. A medicine kept in heat, humidity or light may degrade before its printed date, while correct storage helps it last to the date shown. Once a product is opened, especially liquids, eye drops and creams, it often has a shorter usable period than the unopened expiry suggests.
Because reduced potency can be risky for important medicines, the safest practice is to replace expired products rather than use them, and to dispose of old medicines through a pharmacy. If you are unsure whether an expired medicine is still safe and effective, ask your pharmacist.