What could cause a sudden drop in b12 levels in 5 months600 to 371. extreme stomach pain 20weightloss , mri ct colonoscopy can't find cause. what else?

Functional loss: The tests you undertook look for anatomical problems that would correlate with B12 deficiency. But unless you've had surgery recently, the chances of an anatomical problem are remote. More likely you have a functional loss of B12. Either your diet has changed or you have developed problems with B12 absorption like achlorhydria or intrinsic factor (IF) deficiency which are found by biochemical test