❓My vitamin B12 came back low on a checkup — what should I do, and is it serious?

A number that's sitting right at the lower limit but still within normal range is the kind of thing that deserves attention but not panic. I'd follow the doctor's advice and retest, and it's also worth adding homocysteine or MMA (methylmalonic acid) to the panel — these are more sensitive markers, since plain serum B12 can sometimes mask an actual cellular-level deficiency. At this stage, most people can bring their numbers back into a comfortable range within a few months through oral supplements plus regularly eating fortified foods — this isn't the kind of thing that's hard to reverse. What's actually troublesome is ignoring it entirely for a long time and only dealing with it once neurological symptoms like numbness or tingling in the hands and feet show up — recovery from that is much slower. You've caught it now and are starting to supplement, which is the right direction — no need to be too anxious.