From what I understand, this can vary from person to person. I take a B12 injection every month. I have heard of people needing them every two weeks. Mine was due to B12 anemia, which the doctor felt indicated something wrong in my digestive tract since I should get enough B12 from my diet. The shots help with the fatigue and I never located a digestive issue, even after several different tests.
I also get a B12 show every month. It takes around 3 months to kick in, then you start feeling the difference.
How can a shot take 3 months to kick in?
How does this work; it makes no sense?!