New advice on vitamin B12 monitoring in patients taking metformin

Patients on metformin treatment with risk factors for vitamin B12 deficiency should have their serum levels checked periodically, the MHRA advises.

Purple, orange, blue and yellow blood test tubes on a tray.
Patients found to be vitamin B12 deficient should be treated as soon as possible to avoid the development of permanent symptoms. | GETTY IMAGES

Vitamin B12 deficiency is now listed as a common side effect of metformin after Glucophage manufacturer, Merck Serono, reviewed the adverse reaction and found that it may affect up to 1 in 10 patients who take metformin.

Risk factors for vitamin B12 deficiency

Healthcare professionals are advised that deficiency increases with higher metformin doses, longer duration, and in patients with risk factors for vitamin B12 deficiency, including:

  • baseline vitamin B12 levels at the lower end of the normal range, 
  • conditions associated with reduced vitamin B12 absorption eg, in elderly people and those with gastrointestinal disorders, 
  • diets with reduced sources of vitamin B12 eg, strict vegan and vegetarian diets, 
  • concomitant medication known to impair vitamin B12 absorption eg, proton pump inhibitors and colchicine, or
  • genetic predisposition to vitamin B12 deficiency eg, intrinsic factor receptor deficiency and transcobalamin II deficiency. 

Recommendations for healthcare professionals

The MHRA advises that if patients present with suspected symptoms such as megaloblastic anaemia or new-onset neuropathy, vitamin B12 serum levels should be tested, and patients with any of the risk factors above should be monitored for the deficiency periodically. 

Healthcare professionals are reminded that vitamin B12 deficiency can also be asymptomatic. In addition to symptoms of megaloblastic anaemia and neuropathy, other symptoms may include mental health disturbances, glossitis, mouth ulcers, and visual and motor disturbances. 

Metformin treatment should be continued for as long as it is tolerated and not contraindicated, and appropriate corrective treatment for vitamin B12 deficiency is provided in line with current clinical guidelines.

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