Vitamin B12: a real fix for a real deficiency, and nothing more

Vitamin B12 is the clearest example of a supplement that is genuinely important for some people and completely pointless for most. The nutrient is non-negotiable — it’s required to make red blood cells and to maintain the myelin sheaths around your nerves — but your body needs only micrograms of it, and a normal omnivorous diet plus a functioning gut supplies that easily. The entire question is not “should I take B12” but “am I one of the people who can’t absorb or obtain enough.”

What makes B12 different from most supplements is the asymmetry of getting it wrong. Over-supplementing a replete person does nothing — B12 is water-soluble and the excess is excreted. But missing a real deficiency is a genuine harm, because prolonged B12 deficiency damages nerves, and that damage can become permanent even after you finally correct the level. That’s the reason this entry earns a “consider” rather than a “skip”: the downside of inattention is much worse than the cost of a test.

Who is actually at risk

A handful of well-defined groups account for nearly all real deficiency:

• Vegans and strict vegetarians. B12 is essentially absent from plant foods. This is the one group for whom routine supplementation (or fortified foods) is a clear yes, not a maybe.
• Older adults. Absorption of food-bound B12 declines with age as stomach acid drops, so a meaningful fraction of people over 60 under-absorb it even on an adequate diet.
• Long-term metformin users. Metformin reduces B12 absorption and lowers serum concentrations over years of use.
• Long-term proton-pump-inhibitor users. PPIs suppress the gastric acid needed to release B12 from food, and are independently linked to deficiency; the metformin-plus-PPI combination, common in diabetics with reflux, compounds the risk.
• People with pernicious anemia or malabsorption (celiac, Crohn’s, gastric surgery), who lack the machinery to absorb it at all and typically need injections or high-dose oral B12.

Test, then correct

Because supplementing a replete person is inert, the sensible move is to measure rather than assume. A serum B12 level is the first-line test; when it’s borderline, methylmalonic acid (MMA) is a more sensitive confirmatory marker of true tissue deficiency. Watch for the neurologic tells — numbness or tingling in the hands and feet, balance problems, cognitive fog — because these can appear before, or without, the classic anemia, and they’re the symptoms you most want to catch early.

If you are deficient, correction is easy and cheap: oral B12 works for most causes (higher doses for malabsorption, since absorption is inefficient), and injections are reserved for pernicious anemia or severe deficiency. Once your level is repaired, more is not better — there’s no reserve to build past full.

The honest bottom line

B12 is a targeted repair, not a daily optimizer. If you’re plant-based, older, or on long-term metformin or a PPI, get a level checked and correct a real deficiency — the neurologic stakes make that worthwhile. If you eat animal foods, have a healthy gut, and aren’t on those drugs, a B12 supplement is almost certainly doing nothing measurable for you. The value here is entirely in identifying the right people, not in the pill itself.