Complexities of the interlocking systems that make up the BBB to the side, it's worth knowing that most B12 supplements come as cyanocobalamin, which requires methylation (and so becoming methylcobalamin) before it can be used to metabolize homocysteine. This matters because just two SNPs determine how effective any of us might be at methylation, so about 3/4 of all of us are suboptimal and 1/4 of us are pretty terrible at making this conversion. Methylation's important for B6 and folate as well.
So, either know your status on those two SNPs (google MTHFR for more details), have your homeocysteine tested (surprisingly no longer considered standard of care so you have to ask) or load up on methyl donors in your diet. The most common of those is lecithin, and the most dense source is egg yolks. All that avoiding egg yolks because cholesterol? Yeah, that didn't help. There are lecithin supplements (from soy or sunflower seeds) and other methyl-donor supplements like betaine (trimethylglycine).