Warning!
I am not a medical professional.
However, I asked my favorite doctor and she seemed to think it would be okay.
She said rust would just look like iron to your body and it would be consumed like food.
So, I guess it is broadly safe.
She also said that she wouldn't do it under any circumstances.
If the container is rusting so extensively you swallow sharp flakes of metal, you can die really horribly.
outdoors.stackexchange.com/users/767/theJollySin
People do not realize that their public water are delivered by iron pipes buried 20 to 80 years ago.
I was an engineering student and if you cut those pipes you will see rust around the pipes.
So people do not realized that they are drinking water through rusted interior of water pipes.
No one has died from it.
outdoors.stackexchange.com/users/5528/steve
Rust is not harmful to consume in either form (red or black)
Black rust is magnetite and is what makes cast iron cookware black.
What is dangerous is being cut by something rusty, and danger has nothing to do with the rust itself.
It is simply a great place for tetanus bacteria to live.
outdoors.stackexchange.com/users/5565/Escoce
I'd be wary of drinking from anything rusty personally - I'm not aware of the type of metal your thermos is made from, but several can start to produce potentially poisonous chemicals when they begin to oxidise.
Sure, you could be ok
but I wouldn't say it's worth the risk.
In terms of cleaning it, try something like Zud cleanser (readily available in the UK, not sure about other parts of the world.)
Does a really good job at removing rust on anything I've tried it with!
outdoors.stackexchange.com/users/74/berry120
Put dish wash powder in and fill with hot water it will fizz let sit over night (don't put cap on gases
will blow it off)
was out it will look like new.
outdoors.stackexchange.com/users/2907/user2907
It's harmless.
Rinse out anything loose.
If you want, add a handfull of gravel, a cup of water, and shake for 10 minutes to get stuff out.
A thermos is going to be food grade metal.
So the alloys will not be exotic ones with chromium or vanadium in quantity.
In passing: a 1 or 2 liter bottle with a pair of heavy socks pulled over it works nicely as a cheap thermos.
I've used this for coffee on all day orienteering in winter, and even by day's end the coffee is at least warm.
outdoors.stackexchange.com/users/2157/Sherwood Botsford
I've got a thermos for coffee and/or water storage that I use on hikes and for bike touring since it keeps things hot for hours.  However, it seems like the inside is rusting a bit.    Is this safe to use as is? Is this easily remedied, and how can I protect it from rusting again?
Is it safe to drink out of a rusty thermos?
outdoors.stackexchange.com
<safety><food><water><maintenance>
outdoors.stackexchange.com/questions/3367
outdoors.stackexchange.com/users/429/Benzo
While some say that rust is not harmful to consume, it is best to avoid swallowing sharp flakes of metal or being cut. Some metals may produce poisonous chemicals when they start to oxidise. Clean it by adding dishwasher powder and hot water, Zud cleanser, or add some gravel and water and shake for 10 minutes to get rid of loose flakes. Rust is not harmful in itself but drinking sharp metal flakes is. You can clean and rinse these out.
Rust itself is not harmful. To avoid drinking sharp metal flakes, clean and rinse out anything loose.