It is known among the medical profession that long term use of metformin can cause vitamin B12 deficiency. Below is the link to the British Medical Journal article.
http://www.bmj.com/content/340/bmj.c2181
The symptoms are the same as pernicious aenemia, and often mistaken for complications of diabetes. Studies have shown that up to 30% of those who have taken metformin for four years have low B12, low enough to affect their general health. The percentage increases for longer term users. I was on metformin for seven years before experiencing B12 deficiency.
Metformin disrupts the body's ability to use B12 from food, particularly the part of the process that involves calcium so the treatment are either injections of B12 or a B12 supplement taken with a calcium supplement.
I've spoken to both my GP and Endocrinologist about this and the excuse I've been given as to why they don't routinely test for B12 deficiency among those who have been on metformin long term is that it's 'uncommon'. 30% isn't uncommon in my opinion, especially when there is a risk of permanent nerve damage if left untreated. My endocrinologist even told my that I couldn't have a B12 deficiency because my blood count was normal, he didn't understand that the aenemia caused by low B12 doesn't reduce the blood count it just changes the shape of the blood cells. I unapologetically enlightened him on the fact that the aenemia caused by low B12 is a megloblastic aenemia.
Some clues to B12 deficiency that are undeniable are the loss of the 'moons' at the base of you fingernails. A year later mine are only just becoming visible again. Unexplained vision changes, my vision correction went from +.25 to +1.50 in six months, a year later it's returned to normal. Persistent diarrhea, as though the most unwelcome side effect of metfromin had returned with a vengence. My GP thought I might have IBS, but within 24 hours of starting B12 the diarrhea was gone and hasn't returned. Very dry skin, even in the summer, that took about a month to resolve.
Many of the other symptoms are easily confused with other conditions -- tiredness, joint and muscle pain, mood swings, erratic BG readings, loss of sensation in hands and feet, clumsiness, confusion, depression, etc.
If you have been on metformin long term, or have any of those symptoms, it's worth asking for a blood test to rule out B12 deficiency. If you do have it, it's easily treated. There have been reports of diabetic neuropathy improving with B12 treatment, but I suspect that the neuropathy was really undiagnosed B12 deficiency.
