This video covers metformin and decreased serum vitamin B12. This was a study nested within NHANES, the National Health And Nutrition Examination study. They did confirm a decrease in serum levels of vitamin B12.

Other studies have shown that it takes about 4 years to see this impact. Government and other standards committees, such as the US Preventive Services Task Force DON'T have recommendations re: this issue. That's because this appears to be a "biochemical" deficiency. In other words, it's not clear that this is related to anemia, depression, neuropathy, or other significant problems.

Normal supplements did not impact these levels. Higher concentration, specific supplementation, such as B12 injections did impact the deficiencies.

https://care.diabetesjournals.org/content/35/2/327.figures-only