The short answer to this question is NO. It is not recommended to put a thermos in the microwave, as most thermoses are made with stainless steel.
If you put stainless steel in the microwave, it could cause damage to the microwave. Also, it could cause both the microwave and the thermos to catch on fire!
Thankfully, there are other ways to heat a thermos; let’s explore the options.
How To Heat A Thermos Properly:
Here are the steps to preheat your thermos:
Boil some water on the stove or in the microwave in a separate container.
Pour the boiling water into your clean and empty thermos until completely full.
Be careful when pouring the water into the thermos, so you do not burn yourself!
Place the lid on the thermos and leave it to sit for 5-10 minutes.
Remove the lid and dump out the water.
Dry the inside of the thermos quickly with a towel. Be careful when doing this as the metal inside the thermos is extremely hot!
Safely pour heated food or drink into the thermos and replace the lid.
Enjoy your hot meal or drink whenever you please!
Preheating your thermos seems to be the only way to ensure your food or drink stays hot for longer than a couple of hours.
Depending on what thermos you buy, your food or drink could stay hot, or at least very warm, for anywhere between 5 hours to 14 hours!
With these things in mind, it is safe to say the best way to heat a thermos is by preheating it with boiling water, then adding your hot food or beverage of choice.