A B12 shot is administered by injecting vitamin B12 directly into muscle tissue, typically in the upper arm, thigh, or hip
After first entry, an opened repeated-entry vial is generally dated and discarded within 28 days unless the manufacturer states another period.
This is what I use on clients with thinning hair due to tight ponytails, use of hot tools, chemical processing, extensions and hormonal imbalances, says Knight
The specific open question is copper accumulation
It is tempting to just say not very much
For example, many oral versions of peptides (like BPC-157 and Thymosin Beta 4) are not bannedits just the injectables