A study published in the April 2016 Journal of Clinical Endocrinology and Metabolism looked at more than 1,000 subjects who had been taking metformin for about 12 years

Other groups who may benefit include: Patients following gastric surgery (gastrectomy, bariatric procedures) where intrinsic factor production or absorption sites are compromised Individuals with malabsorption disorders such as Crohn's disease, coeliac disease, or chronic pancreatitis Strict vegans and vegetarians with dietary insufficiency, though oral supplementation is usually tried first Elderly patients with atrophic gastritis or reduced gastric acid production Patients on long-term metformin or proton pump inhibitors, which can impair B12 absorption [6] [8] Individuals with nitrous oxide exposure (recreational or medical), which can cause functional B12 deficiency Diagnosis typically involves serum B12 measurement, with levels below approximately 148 pmol/L (though thresholds vary between laboratories) generally indicating deficiency
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If the injection is needed (e.g., to replete B or B in a deficient mother), it can be used, but the infant should be observed for any unusual symptoms (though none would be expected from B vitamins, apart from perhaps looser stools if anything)
Older Adults Aging can lead to decreased stomach acid production, impacting the absorption of B12 from food sources