injectable vitamin b12 is considered a dietary supplement Compounded Cyanocobalamin (Vitamin B12) Injection dogs and horses as needed B12 Shots: Benefits, Uses, Safety,
Description
Such finding was ultimately confirmed by a later study conducted by the same team, investigating the clinical correlates of Vit B12 levels in the cerebrospinal fluid (CSF)

Studies have found that supplementation with B12 can help to regulate normal sleep-wake patterns
GTEx Consortium

These may include minor side effects such as a little redness, swelling, or slight bruising at the injection site, which typically resolve quickly
