It is a known side effect of metformin, but supposedly not a problem if you have a normal healthy intake of Vitamin B-12, or take a vitamin B supplement.
I have just extracted this from Wikipedia:
"Metformin may reduce serum folic acid and vitamin B-12 levels. These changes can lead to hyperhomocysteinemia, adding to the risk of cardiovascular disease in people with diabetes. There are also rare reports of megaloblastic anemia in people who have taken metformin for 5 years or more. Reduced serum levels of vitamin B-12 occur in up to 30% of people taking metformin chronically. However, clinically significant deficiency is not likely to develop if dietary intake of vitamin B-12 is adequate. Deficiency can be corrected with vitamin B-12 supplements even if metformin is continued. The metformin-induced malabsorption of vitamin B-12 is reversible by oral calcium supplementation. The general clinical significance of metformin upon B-12 levels is as yet unknown."
