Метформин-индуцированный дефицит кобаламина (вит. В12)
 

Pекомендации по диагностике и ведению данной проблемы в недавней публикации:

Diabetes Res Clin Pract. 2012 Jul 7.
Recommendations for diagnosis and management of metformin-induced vitamin B12 (Cbl) deficiency.
Mazokopakis EE, Starakis IK.
Department of Internal Medicine, Naval Hospital of Crete, 73 200 Chania, Crete, Greece.

Metformin treatment is a known pharmacological cause of vitamin B12 (Cbl) deficiency with controversial responsible mechanisms. A possible diagnosis of this deficiency is based mainly on the combination of patient's medical history (usually long-term metformin use), clinical examination (possible neuropsychiatric symptoms and signs), laboratory studies which confirm a Cbl deficiency (haematological abnormalities, low serum Cbl levels, elevated serum total homocysteine and methylmalonic acid levels), and exclusion other causes of Cbl deficiency (as pernicious anaemia, food-cobalamin malabsorption syndrome, other drugs, etc.). In our review, recommendations for diagnosis and management of metformin-induced Cbl deficiency (MICD) in diabetic patients based on medical bibliography are presented and discussed.

Альтернативный подход - ежегодная профилактика В12-дефицита в др. полнотекстовой статье:

J Young Pharm. 2010 Oct;2(4):428-9.
Revisiting Metformin: Annual Vitamin B12 Supplementation may become Mandatory with Long-Term Metformin Use

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Или добавление кальция к метформину может препятствовать риску развития кобаламино-дефицита:

Diabetes Care. 2000 Sep;23(9):1227-31.
Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin.

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Некоторый апдейт по теме: похоже, что метформин просто снижает концентрацию кобаламина в крови, усиливая его депонирование в клетках, а не вызывает дефицит В12 в организме, подробнее:

Metformin (Glucophage) is regarded a first-line drug in the treatment of type 2 diabetes mellitus. However, it has been shown that treatment with metformin is associated with a decrease in serum vitamin B12 concentrations, which worsens with increasing dose and length of treatment (Ting et al., 2006; de Jager et al., 2010; Liu et al., 2014). The association has been suggested to be due to interference with the calcium dependent intestinal uptake of the vitamin B12-intrinsic factor complex (Bauman et al., 2000). However, although serum B12 is reduced, metformin treatment has been associated with reduction in plasma MMA and indications of improved intracellular B12 status in diabetics (Obeid et al., 2013). Hence, a vitamin B12 resistance phenomenon was suggested in diabetics, and metformin treatment seemed to improve the intracellular vitamin B12 metabolism, contrary to previous beliefs. Two studies found that patients treated with metformin had reduced levels of both serum vitamin B12 and haptocorrin, but treatment was without effect on the levels of holo-TC, the bioactive form of vitamin B12 that is distributed systemically to all cells in the body (Leung et al., 2010; Greibe et al., 2013c). Another study showed that treatment of rats with metformin increased hepatic accumulation of vitamin B12 (Greibe et al., 2013b). This suggests that metformin may alter the homeostasis and tissue distribution of vitamin B12, the consequences of which remain to be investigated. Because not all cases of low serum vitamin B12 imply deficiency, and normal and high serum levels of vitamin B12 are likewise observed in patients with severe functional vitamin B12 deficiency, concern has been raised regarding the interpretation of the effects of metformin on vitamin B12 status (Obeid, 2014). In terms of prevention, diabetics receiving metformin should be regarded at increased risk, and routine screening and supplementation if necessary, should be recommended (Valdes-Ramos et al., 2015).
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Front Mol Biosci. 2016 Jun 27;3:27.
Biomarkers and Algorithms for the Diagnosis of Vitamin B12 Deficiency.

Здравствуйте. Я обследую некоторых пациентов на дефицит витамина 12 в на метформине и часто дефицит выявляется. Но я правильно поняла, что достаточно 2 ампул (1мг)витамина в год и все?

Да, такая мысль о профилактике на фоне постоянного лечения метформином была высказана, например, здесь:
Metformin-induced Vitamin B12 Deficiency Presenting as a Peripheral Neuropathy - David S.H. Bell, MD - South Med J. 2010;103(3):265-267 ([Ссылки могут видеть только зарегистрированные пользователи. ]):