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.
No countdown to draw — Antihistamine tablets (Benadryl, Zyrtec, Claritin, Allegra) goes by the date on the package, not by when you opened it.
Published storage guidance, quoted from ALLEGRA ALLERGY (fexofenadine hydrochloride) label, via DailyMed (NIH/NLM) and U.S. Food and Drug Administration:
Air and warmth are what shorten an opened item's life — the right container buys back the days the packaging gives up once the seal's broken.
Keep Antihistamine tablets (Benadryl, Zyrtec, Claritin, Allegra) fresh longer ↗ Need a fresh one? ↗
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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. Published guidance, by source: ALLEGRA ALLERGY (fexofenadine hydrochloride) label, via DailyMed (NIH/NLM) — Store between 68–77°F (20–25°C); Protect from excessive moisture; Do not use if the blister or foil seal is torn or open. U.S. Food and Drug Administration — Keep it out of a damp bathroom cabinet; Go by the printed expiration date on the label or carton. These label quotes are from the Allegra Allergy (fexofenadine hydrochloride) label only. Benadryl (diphenhydramine), Zyrtec (cetirizine) and Claritin (loratadine) are different drugs with their own labels — read the carton you actually have. The FDA lines are general guidance for medicines. Check the printed date and ask a pharmacist.
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.