Vitamin B12 and medications.

Vitamin B12 is required for nerve function, DNA synthesis, red blood cell production, and energy metabolism, and a shortfall shows up neurologically as well as in anaemia. Those are roles the vitamin plays in the body rather than benefits a supplement adds on top: the case for taking it rests on correcting a shortfall, not on topping up someone who already has enough, so it is not an energy supplement for a replete person and not a treatment for anaemia. The methylcobalamin form is the active one and is generally preferred, especially for nerve health, with a usual dose of 500 to 1,000mcg. Some groups need it more than others: vegans and vegetarians get essentially none from plant foods, absorption declines with age so sublingual forms suit the over-60s, and two common medicines can lower it. Metformin lowers B12 over time in some long-term users, and proton pump inhibitors such as omeprazole and lansoprazole can reduce how much B12 you absorb from food. It pairs with folate and TMG in the methylation cycle. The one trial here that tested B12 on its own was in people with diabetic nerve damage who were already low in B12, where it improved nerve conduction; the brain-atrophy trial often quoted alongside it gave three B vitamins together, so nothing in it can be put down to B12 alone. It is extremely well tolerated, with any excess simply passed in urine. If you take metformin or a PPI long term, checking your level rather than assuming you are fine is the sensible move.