vitamin b12 and vitamin c injection side effects Injection: Benefits Risks Vitamin B12 Deficiency Anemia: Symptoms,

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[20] For nutritional deficiency without intrinsic-factor loss, shorter courses-such as 30-100 mcg intramuscularly daily for five to ten days followed by 100-200 mcg monthly-are often sufficient

ROBINS-I A Tool Assessing Risk Bias Non-randomised Studies Interventions 355, i4919

Aseptic Preparation and Storage: Principles for Clinical Trial Settings Reconstitution of injectable investigational products must be performed by trained personnel following the trial protocol, using the specified diluent, in an appropriate aseptic environment

So its important to supplement in some way if youre not getting enough vitamin B12 through your diet alone