Metformin and Vitamin B deficiency

I found the original publications in BMJ (link here) about risk of folic acid deficiency for metformin users, and had a sigh of relief.  It is not as bad as it sound in the popular press.  The difference in concentration between people who  take metformin and  those who don't take is only about 5% - still important for us who are trying to conceive to take not, but nothing to worry about if we have a healthy diet and a folic acid supplement. 

The risk for vitamin B12 (thiamine) deficiency is higher though. I found this paragraph from the article most relevant to people with PCOS:

"There are few disadvantages to the use of metformin. Metformin does, however, induce vitamin B-12 malabsorption, which may increase the risk of developing vitamin B-12 deficiency5 6 7—a clinically important and treatable condition. In addition, metformin treatment has been reported to be associated with decreased folate concentration, although the mechanism of this effect has not been elucidated.8 Finally, decreases in both folate and vitamin B-12 concentrations might, in turn, result in an increase in homocysteine concentrations (web figure A), an independent risk factor for cardiovascular disease, especially among individuals with type 2 diabetes"

Basically it says, keep your vitamin B levels high while you are on metformin! Having PCOS already increase your risk of heart problems - don't add to it.  And the longer you take metformin, the higher your risk of low B12 levels, because metformin stopped it from being properly absorbed.