I'm glad you were able to figure it out. Something similar happened to me with B6, but I didn't know about B12 toxicity.

I think I remember being told that excess B12 just goes out of your body with urine so toxicity isn't usually a concern, but I guess you're right and without the enzyme to help your body process it you could easily fall behind when taking a supplement.

Excess B6 is more well-known for causing the same symptoms as low B12. In my situation, my neuropathy had improved and then slowly started getting worse and we didn't know why. When my B levels were tested, my B6 was high, though I wasn't knowingly supplementing. It turned out I was drinking one daily Liquid IV (the electrolyte supplement) because I have POTS, and electrolyte supplements are one way to manage the symptoms. Liquid IV is supplemented with B12 and B6, and I didn't even know it. It's 140% the recommended daily value. Thankfully, I switched electrolyte supplements, and my neuropathy went back to baseline.

Now, one of my pet peeves (as someone who needs to use electrolyte/hydration supplements daily) is the proliferation of all the different formulations that have all this extra stuff in it and artificial sugar when the WHO formulation for an oral rehydration solution requires real sugar and doesn't require the extra vitamins/minerals. But that's an argument for another post >_

I’m glad the doctor ordered the appropriate test to determine the cause of the problem before throwing more meds at the situation! Feel better.