A bit of closure on this thread: I initially posted because I'd found I had a voracious need for B12, and I hoped to find the cause. Well after several years of searching, I finally found it.
I'm now certain my B12 hunger was driven by mercury toxicity. In late 2016 I started the "Core Four" nutrients (C, E, magnesium, zinc) recommended for the Andrew Cutler chelation regimen. The Core Four is designed to be protective against mercury oxidation and to help repair some damage. Instantly, literally overnight, my B12 needs dropped in half. Several other things happened that also suggested mercury as the cause. That gave me the incentive to launch a fairly expensive round of dental work.
In 2017 I gradually got all my amalgam fillings removed. My symptoms and my B12 needs gradually decreased as the mercury came out. By the time the last amalgam was gone, I had almost no symptoms and almost no need for B12. That continued to improve, and as of today I haven't used any supplemental B12 for about 3 months. That is a dramatic improvement from 2 years ago, when I had to take 5-7mg per day or suffer every day and night! (That's 5-7mg/day of transdermal oil, which is supposed to be 80% absorbed. I used to take 25-30mg/day of sublinguals and it didn't work as well as the oil.)
When I started the Cutler chelation process, some of my symptoms returned, and B12 didn't help much. I've just discovered that milk thistle (which supports the liver) helps a lot. Apparently the chelation stirs up the mercury stored in my body, and my liver suffers the brunt of disposing of it. The symptoms were nearly identical to some of my original B12 symptoms, so it may be that liver distress was a problem all along.
I hope this helps someone else in a similar situation...
