You should carry in your hand baggage. You will need a letter from your GP stating that you need to carry the medication and a list of your current prescription medications. TBH I’ve never been asked for it by either UK or US customs but you need it just in case.If your GP is not prescribing then you will have to put it in your checked luggage and hope for the best.
I'm also traveling and if I put it in checked bag. No harm can get ampoules. I also want to take all of my syringes and needles. I have doctor note but since I'm dual citizen may my doctor note not to be acceptable by all airlines. It's really a dilemma.
I can't speak for international travel, but I just did a cross-country, round trip flight within the US with hydroxocobalamin from Germany and insulin syringes. I put everything in my carry-on bag and had zero issues. I did not have a doctor's note and nobody asked. Your mileage may vary, especially since you are traveling international, but that was my experience.
I live in the US and found this on our TSA website. It doesn’t specifically mention international travelers though. I would be sure everything is in original boxes/containers and declare it when you are questioned at customs.
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