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 — Antihistamine tablets (Benadryl, Zyrtec, Claritin, Allegra) carries a printed expiration date and that's the date to go by. Go by the printed expiration date. Tablets in intact blister packs are the best protected from humidity — allergy medicine kept in a car or a bathroom loses potency faster than the date suggests. A less effective antihistamine won't hurt you, but it may simply stop working. Not medical advice — follow the label and ask a pharmacist.
Room temperature 68–77°F (20–25°C), dry, tightly closed in the original blister pack or bottle. 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 or discoloured tablets; Blister packs that have been punctured or wet; An off smell; Past the printed expiration date. Not medical advice — ask a pharmacist if you're unsure.