I do consider from time to time if I need more mb12, but I have chemical sensitivities whether or not I take mB12. I do not believe mb12 will affect that for me but I think tha could be an artifact of my cbs +/+ genes and possibly something downstream would help. By comparison all the methylation supplements I take have a very small impact that I can feel.
Try this - get a bottle of Douglas Labs liquid methylcobalamin. It's not that expensive. Take one drop (or if you're really brave a whole dropperful (1000mcg), but I don't recommend it). Hold it under your tongue for 30 seconds to allow it to absorb.
Either it will make you feel better, or if your CBS is activated, you will feel excitotoxicity/fight or flight (stressed, wired, nervous, etc.) It might take a few hours or days to kick in.
I was making the same mistake you are. B12 in a pill form - nothing. Sublingual B12 - bam!
I don't think you have to measure anything. Your symptoms are telltale enough, and serum B12 isn't really reliable anyway.
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Ok, how does B12 help you make glutathione? B12 doesn't make it directly but it's at the start of a chain of things that make it. So first you have methylfolate, then you have methylcobalamin. These work together synergistically to make methyl groups. These convert into methionine, then SAMe, which gives up methyl groups to help with DNA regulation and hundreds of other very important jobs. If this part was working correctly you wouldn't need the DHEA. Then homocysteine is created. This is at the top of the transsulfuration pathway where CBS is.
If this pathway is working correctly, then you'll make some taurine, but more glutathione. At this point, your chemical sensitivities should start to get better.
So what you're doing is taking all these things farther along the cycle (needed because your cycle is not working correctly), but missing the really big one at the begining of the cycle (the B12). If you start taking some sublingual or injected B12, your need for the DHEA, cysteine, etc. should diminish, and your chemical sensitivities should get better.
ps. you also might want to back off at least somewhat on the methylfolate until you get handle on the B12. You should be taking more B12 than methylfolate to avoid methyl trapping. The methyl trapping will stop methylation - the opposite of what you want.