The reason is that B12 absorption is a complex process with multiple steps involved and which decline with age
Overall, the route is chemically plausible, concise, and complements existing literature strategies
Reported side effects include mild headaches or dizziness (more common when combined with growth hormone peptides), injection site irritation, GI discomfort from low-quality oral preparations, and occasional mood or energy changes
All three published full-paper human studies include Edwin Lee as lead author
Anyone outside a healthy, monitored research context and especially anyone in one of the sensitive groups above sits entirely beyond the edge of what has been studied, which is exactly why a clinicians judgement, not a web page, is the right place for that decision to be made