 [ Symptoms ] how long does a b12 shot last i have just received a b12 shot from my doctor how long can i expect the results to last [ Disease ] hi if you see your doctor and work with him you could be on the wrong hormone so it would mean coming off what your on give it a few days and get a blood test from this they should be able to tell which your low on and re start from there and see how you go then do hope this helps good luck