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 — Children's liquid pain reliever (acetaminophen or ibuprofen suspension) carries a printed expiration date and that's the date to go by. Use by the expiration date printed on the carton or bottle, and shake well before every dose. Don't refrigerate unless the label says to. An OTC suspension is not the same as a pharmacist-mixed antibiotic — those DO carry a short after-mixing date (often 10–14 days) printed on the pharmacy label, so always read that label. Not medical advice — dosing for children is weight-based; follow the label and ask your pharmacist or paediatrician.
Room temperature 68–77°F (20–25°C), tightly capped, upright, away from heat and light — do not freeze. 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: Separation or settling that won't shake back in; Colour change; Crystals or grittiness; Past the printed expiration date. Not medical advice — ask a pharmacist if you're unsure.