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 — Robitussin (cough syrup) goes by the date on the package, not by when you opened it.
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 Robitussin (cough syrup) fresh longer ↗ Need a fresh one? ↗
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Yes — Robitussin (cough syrup) carries a printed expiration date and that's the date to go by. Use it by the expiration date printed on the bottle — for most OTC liquid cough syrups, opening does not start a short countdown the way a pharmacist-mixed antibiotic suspension does. Keep it tightly capped at room temperature. Not medical advice — follow the label and ask a pharmacist; don't take medicine that's expired or has changed in colour, smell, or texture.
Room temperature 68–77°F (20–25°C), tightly capped, away from heat and moisture — not the bathroom cabinet. 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. Check the printed date and ask a pharmacist.
Throw it out if you notice: Discoloration; Changed or off smell; Separation or cloudiness that won't shake back in; Past the printed expiration date. Not medical advice — ask a pharmacist if you're unsure.