A patient with unexplained fatigue should be worked up, not injected masking a B12 deficiency of unclear cause, or an unexamined thyroid problem, with a monthly shot is a real clinical risk
Administration happened under supervision with precise measurement

Berkeley Heights, NJ 07922 Revised December 2023 PIN350-WES/6 NDC 0143-9621-01 1 mL Vial Cyanocobalamin Injection, USP Rx only 1,000 mcg/mL For IM or SC use ONLY Protect from light Contains Benzyl Alcohol NDC 0143-9621-25 Rx only Cyanocobalamin Injection, USP 1,000 mcg/mL For Intramuscular or Subcutaneous use ONLY Contains Benzyl Alcohol as a Preservative 25 x 1 mL Vials NDC 0143-9620-01 10 mL Multiple Dose Vial Cyanocobalamin Injection, USP Rx only 10,000 mcg per 10 mL (1,000 mcg/mL) For Intramuscular or Subcutaneous use ONLY Contains Benzyl Alcohol as a Preservative NDC 0143-9620-10 Rx only Cyanocobalamin Injection, USP 10,000 mcg per 10 mL (1,000 mcg/mL) For Intramuscular or Subcutaneous use ONLY Contains Benzyl Alcohol as a Preservative 10 x 10 mL Multiple Dose Vials NDC 0143-9619-01 30 mL Multiple Dose Vial Cyanocobalamin Injection, USP Rx only 30,000 mcg per 30 mL (1,000 mcg/mL) For Intramuscular or Subcutaneous use ONLY Contains Benzyl Alcohol as a Preservative NDC 0143-9619-10 Rx only Cyanocobalamin Injection, USP 30,000 mcg per 30 mL (1,000 mcg/mL) For Intramuscular or Subcutaneous use ONLY Contains Benzyl Alcohol as a Preservative 10 x 30 mL Multiple Dose Vials NDC 0143-9619-01 Rx only Cyanocobalamin Injection, USP 30,000 mcg per 30 mL (1,000 mcg/mL) For Intramuscular or Subcutaneous use ONLY Contains Benzyl Alcohol as a Preservative 1 x 30 mL Multiple Dose Vials

The evidence for meaningful additional benefit from niacinamide inclusion is limited, but like glycine, it adds minimal cost and no significant risk
Why are people opting for this injection away from the NHS
Which combination (if any) fits you is a clinical decision, not a guess