Go by the expiration date printed on the package. For most OTC medicines, opening the container doesn't start a shorter countdown — storage is what actually shortens a medicine's life.
Yes — Acetaminophen tablets (Tylenol) carries a printed expiration date and that's the date to go by. Go by the expiration date printed on the bottle. Opening the bottle does not start a separate countdown, but heat and humidity do shorten a medicine's real life — which is why the bathroom is the worst place to keep it. Keep the desiccant packet in the bottle and leave tablets in their original container so the label and date stay with them. Not medical advice — follow the label and ask a pharmacist.
Room temperature 68–77°F (20–25°C), dry, in the original bottle with the cap closed — not the bathroom. Heat and humidity shorten a medicine's real life faster than the printed date suggests, which is why a bathroom cabinet is the worst place for it.
Throw it out if you notice: Crumbling, chipped, or stuck-together tablets; Discoloration or spotting; A vinegary, musty, or chemical smell; Past the printed expiration date. Not medical advice — ask a pharmacist if you're unsure.