For what it’s worth, I used to slam energy drinks full of megadoses of B vitamins and felt no worse for it. Then, read about heart problems and neuropathy being associated with high doses of B supplementation and cut back.
Also, remember I took SAMe in combo with B12, but others with post-drug syndrome have mentioned troubles with B12 alone.
Have you considered starting with an intermediate dose? Something that’s only 10s of times more than the RDA instead of 100s of times more?