Metformin: Risk of Vitamin B12 Deficiency

Drug Safety Update

Vitamin B12, also referred to as cobalamin, is a water-soluble vitamin that is naturally present in foods or animal origin such as meat, dairy and eggs. Vitamin B12 deficiency is defined as a reduced serum B12 level of ≤300 pg/mL (221 pmol/L).

Vitamin B12 deficiency can be asymptomatic or associated with haematologic (such as megaloblastic anaemia), neurologic (such as peripheral neuropathy), and psychiatric disorders (such as impaired memory and irritability) that can usually be reversed by early diagnosis and treatment.

Vitamin B12 deficiency is now regarded as a common adverse drug reaction and may affect up to 1 in 10 people who take metformin.

Several oversea studies have shown that the occurrence of vitamin B12 deficiency is common among patients on metformin (ranging from 4.3% to 30%).
The risk increases with a higher dose, longer treatment duration and in patients with risk factors for vitamin B12 deficiency.
In a local study by Krishnan et al. (2020), non-Malay race (Chinese or Indian) was associated with an approximately four-fold increased risk, whereas the duration of metformin use of more than five years conferred a greater than two-fold increased risk for vitamin B12 deficiency.

Recommendations

Perform serum vitamin B12 level tests when metformin-induced vitamin B12 deficiency is suspected.

Consider periodic vitamin B12 monitoring in patients with risk factors.

Educate patients on metformin therapy to seek medical attention if they develop signs and symptoms suggestive of low vitamin B12 level, including new or worsening symptoms of extreme tiredness, a sore and red tongue, pins and needles sensation, or pale or yellow skin.

Treat vitamin B12 deficiency promptly in accordance with the latest clinical guidelines and continue metformin therapy for as long as it is tolerated and not contraindicated.

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