The longer you have been on metformin, the likelier a shortage
At five years, low or borderline B12 was twice as common on metformin as on placebo, 19.1 percent against 9.5, and each extra year raised the odds 13 percent.
What the study found
Every extra year on metformin raised deficiency odds 13 percent
At five years, low or borderline B12 was twice as common on metformin as on placebo, 19.1 percent against 9.5. Neuropathy was more common among the metformin users whose B12 was low. The risk is cumulative, so the people most exposed are the ones who have taken it longest.
The caveat · Nerve damage link only observed
B12 was measured only at years 5 and 13, so nothing is known about the years between. Frank deficiency at 13 years, 7.4 percent against 5.4, did not reach significance. The neuropathy link is an observation within the group, not a demonstration that the low B12 caused it.