It is perfectly safe to wash brand-name thermoses in the dishwasher, and the same applies to most other insulated flasks. However, it is always best to check each brand’s cleansing instructions, as some recommend hand washing only. Thermos—the brand—has made it clear that most of its products are dishwasher-safe and run no risk of significant damage or malfunction [5]. However, they also recommend handwashing if possible, as the dishwasher can sometimes cause cosmetic damage to the flask.