I got tested today and my levels are too high (1,117 pg/mL). And I’m vegetarian AND take lots of metformin.
I’d rather take the risks of too high B12 than the risks of too low B12, so I make sure to get a lot. Many people are deficient, especially in old age or vegetarians or vegan. The risks are real though, there is an increase in lung cancer according to Mendelian randomization. The absolute risk is low though.
We found circulating vitamin B12 to be positively associated with overall lung cancer risk in a dose response fashion (odds ratio for a doubling in B12 [ORlog2B12 ] = 1.15, 95% confidence interval (95%CI) = 1.06-1.25).
Have you checked whether you have foods that are fortified?
I eat nutritional yeast, and vary between fortified and unfortified, and when I eat the fortified my levels can increase a lot, and so I switch back and forth.
You mentioned vegetarian, so perhaps you are eating eggs or dairy that are high in B12?