jillc39 is correct. No injectable forms of B12 are available without a prescription in the US. Also, cyanocobalamin is the version that is most commonly prescribed over here, so even if you get a prescription you might have a little trouble finding a pharmacy that carries hydroxocobalamin. Sorry.
You might also want to think about contacting your travel agent to find out if there would be any restrictions on taking the ampoules on a plane to US - suspect there may be.
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