I take both my B12 and thyroid together. Never had a problem with this as tests show both are doing well.

FOOD, esp the fiber in food complexes the B12 and reduces its passive absorption in the intestine (this is not including intrinsic factor).

Complexes usually have small amounts of B12 and this will not make it to the intestine if food is present and/or you have little or no intrinsic factor activity. If you have good intrinsic factor status, you may absorb the B12 in them. But people who are testing low and/or have neuro symptoms typically have low serum levels, and hence that implies that intrinsic factor is not working. Hence you need to assure passive absorption instead, meaning taking a high dose orally on a empty stomach.

Intrinsic factor needs acidic stomach and can be destroyed by autoimmune disease or be not present genetically .