how often for b12 injections Shots at Home: How, Where & to Inject Yourself Vitamin B12 Injections Clinic Near
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Why should you get B12 injections

In UK practice, monitoring is primarily based on clinical response rather than routine blood tests
Finally, one very recent study in the USA has demonstrated that in some individuals with neurological symptoms, the presence in the blood of antibodies against this receptor may interfere with the proper uptake of B12 into the brain

doi: 10.1038/clpt.2010.57

If they start to dip, it might be time to increase their frequency for a bit