There are a variety of issues with B12. Many depend on your genetic engineering.
You have to do trial and error.
In the US they use the B12 shot that has cyanide in it. What I needed was the version used in Europe that has been broken down somewhat
https://www.mcpiqojournal.org/article/S2542-4548(19)30033-5/fulltext
If you have had a path review done on a CBC you can look for signs of a B12 issues. Hypersegmented neutrophils. But don’t expect your primary care physician to know jack about any of it
Thank you. He wouldn’t for sure. He’d send me to a specialist and charge me for asking.