One important detail I learned during my research into B12 deficiency was -
swallowing a B complex vitamin pill is not much good, because the absorption of B12 from the digestive tract is dependant on the patient having an 'intrinsic factor' in their stomach.
Production of this intrinsic factor reduces with age, (and is totally absent in a person with pernicious anaemia.)
The use of acid supressing medication also inhibits absorption.
There are only 2 ways to be sure of getting enough B12 in your system - bypass digestion by -
1 - injection
2 - absorb through the lining of the mouth
When you consider item 2 and explore how it works, you will be more selective about what you put there!
(Of course this means looking at the ingredients of your toothpaste and mouthwash
)