why are b12 injections better than tablets vs Tablets: What You Need to Know How Quickly Does B12 Shot
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B12 Injections B12 Injections may boost your energy

The NHS treatment page lays it out clearly: Loading phase (no neurological symptoms): 1mg every other day for two weeks, or until symptoms stop improving

One compound reduces energy intake, the other increases energy expenditure at the mitochondrial level
(February 2017)