A. Elevated B-12 levels can be a sign of a serious disease, such as certain types of leukemia or liver disease. If your doctors are unconcerned about your high B-12 levels , it suggests that they have ruled out these other possibilities based other test results (such as white blood cell count or liver enzymes). You could ask them to be sure! Certain drugs used for insomnia (specifically, chloral hydrate) can also cause elevated B-12 levels.
Ermens AA, Vlasveld LT, Lindemans J. Significance of elevated cobalamin (vitamin B12) levels in blood. Clin Biochem. 2003 Nov;36(8):585-90. Link