Metformin-Associated Vitamin B12 Deficiency: An Underrecognized Complication

Metformin-associated vitamin B12 deficiency is a well-established side effect, especially in patients taking higher doses of metformin or who have existing risk factors. Severe deficiency causes a wide range of systemic disorders. Gait instability, which leads to frequent falling, is usually an underrecognized side effect. Older patients are more likely to develop chronic subdural hematoma even with minor trauma. We present a case of 84-year-old man with type 2 diabetes mellitus with acute-on-chronic subdural hematoma from frequent falls. Metformin therapy at dose of 1700 mg/day was given for more than 25 years. He had been in his usual state of health until 10 months ago when he began to have frequent fallings and fatigue. Physical examination in this admission revealed new-onset impaired vibratory sensation and proprioception in both feet and positive Romberg test. Subsequent evaluations demonstrated undetectable plasma vitamin B12 level and elevated plasma homocysteine. Improvement in neurological symptoms occurred within 1 week of vitamin B12 replacement and surgical hematoma evacuation. This case highlights the importance of awareness and periodic monitoring of vitamin B12 status among older patients taking metformin.