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This is usually an indication that you have a methylation defect and need to slow down on the amount of B12 you are taking
Most protocols begin with 1-2 injections weekly for 4-8 weeks
Even though this effect may be mediated by the insulin receptor, experimental KO mice for the insulin receptor gene show a potent glucose lowering effect of IGF-1, indicative that the hypoglycaemic effect is also mediated, in part, by its own IGF-1R
Even if you can absorb B12 but have a deficiency caused by diet, a short course of injections would be the best way to bring levels up quickly, so I imagine that's why you were given it rather than tablets
Many people feel an energy lift within a few days that builds over the following week or two