giving a b12 injection in the arm Injecting What happens when you inject vitamin B12? B12 Shots at Home: How,

Realistic expectations should be calibrated to clinical literature, not testimonials

Severe allergic reactionssuch as difficulty breathing, rash, or swelling of the face and throatare extremely rare but require immediate medical attention

For others, especially those needing nerve or anemia support, improvements might take weeks or months with consistent injections

Science 93:2021

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It is most often based on the clinical symptoms together with laboratory assessment (low serum vitamin B 12 level and elevated serum homocysteine or methylmalonic acid level) and the response to treatment to make definitive diagnosis