I am feeling unwell so am going for blood tests this Saturday morning. B12 is one of the tests. However I received a B12 infusion by IV drip last night (Thursday) because I was unwell and suffering fatigue and migraine. How long should I leave it before the blood tests. How long does B12 stay in the system. I was thinking of having the blood tests this Saturday morning, 36 hours after the B12 drip infusion. Please let me know any ideas or antone's experiences.