There hasn't already been an expiry date on drugs. Where I lived, Law changed maybe 3 decades ago, forcing the manufacturers to print a date.
Light and heat: many organic compounds, biologic or not, undergo reactions around room temperature, with light or without. Food rots, chlorophyll operates, linseed oil hardens in air, and most plastics get destroyed by sunlight. Among the radical reactions that start at room temperature with light, you may know H2+Cl2.