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Question answered:07/01/08 Warning! this question is over two years old.
The SPC for Glucophage [1] reports:
“Very rare: Decrease of vitamin B12 absorption with decrease of serum levels during long-term use of metformin. Consideration of such aetiology is recommended if a patient presents with megaloplastic anaemia.”
In September last year the NLH Q&A Service answered a related question [2] and this highlights a figure of between 10 and 30%.
A 2004 paper (in diabetics not PCOS) [3] assessed the vitamin B12 status of patients with type 2 diabetes who had been receiving metformin treatment for at least one year. This concluded:
“Patients exposed to long-term metformin therapy had 26.7% lower cobalamin, 21.6% lower holotranscobalamin and 9.7% higher HCy serum concentrations than control subjects. Such changes indicate a potential risk for development of vitamin B12 deficiency. Our results highlight the necessity of checking B12 status during metformin therapy.”
References
1) http://emc.medicines.org.uk/emc/assets/c/html/displaydoc.asp?documentid=1043
2) http://www.tripanswers.org/Answer.aspx?QuestionId=2302
3) https://secure.sherbornegibbs.com/bjdvd/pdf/1984.pdf
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