To clean a flask effectively and safely, rinse it right after use, then wash the inside with warm water and a little mild dish soap, using a bottle or flask brush to reach all corners. For deeper cleaning or lingering smells, use a mix of vinegar and water with a spoon or two of baking soda, let it fizz and sit for 10–15 minutes, then scrub and rinse thoroughly and dry upside down with the lid off.

Basic daily cleaning

  • Empty the flask completely and give it a quick rinse with warm water to remove leftover drink and loose residue.
  • Add warm water plus a small drop of mild dish soap, close the lid, and shake or swirl to coat the inside.
  • Use a soft bottle/flask brush if the neck is narrow so you can scrub the bottom and sides without scratching.

Deep clean for stains and smells

  • For metal or typical thermos flasks, fill halfway with warm water and white vinegar (about 1:1), close it, and let it sit 10–15 minutes to loosen buildup and kill odour‑causing bacteria.
  • Add 2–3 tablespoons of baking soda, let it fizz, then scrub with a brush focusing on stained areas and rinse several times with clean water until no smell or residue remains.

Baking soda only method

  • If you do not want to use vinegar, add 2–3 tablespoons of baking soda to the empty flask, then fill with hot (not boiling for some bottles) water, leaving a little space at the top.
  • Let it sit for 15–30 minutes, scrub with a bottle brush, then rinse thoroughly and check that the water runs clear and the smell is gone.

Drying and everyday care

  • After cleaning, always rinse off all soap or cleaning agents and then leave the flask to dry upside down with the cap off so moisture can escape and mould cannot grow.
  • Avoid soaking insulated flasks fully in water for long periods, and clean soon after using coffee, tea, or alcohol to prevent staining and lingering flavour.

Safety notes

  • Do not use harsh chemicals like strong acids or drain cleaners in regular drink flasks; they can damage the material and be unsafe for anything you later drink.
  • If the interior coating is peeling, corroded, or badly damaged even after cleaning, it is safer to retire the flask and replace it rather than keep using it.

Information gathered from public forums or data available on the internet and portrayed here.