DEAR DR. ROACH: I am 73, and I was worried about dementia. Turns out, I am “normal.” However, the neurologist told me to take 2,000 mcg of B12 daily. My multivitamin has 100 mcg, which is 4,167% of the daily value (DV). As I understand the recommended DV is 2.4 mcg, and for those over 50 years old, it is 25-100 mcg.

I raised the question with my oncologist. She said the DVs were developed over 50 years ago and should be ignored. Are the DVs irrelevant, and does 2,000 mcg make sense? — I.F.

ANSWER: The DV (a replacement for what we used to call the recommended daily allowance, or RDA) for vitamin B12 is 2.4 mcg. (There are 1,000 micrograms in a milligram, so 2.4 mcg is a very small amount needed to prevent deficiency.) There is no increased DV for older adults.

This amount of B12 is easily found in nearly all diets, with the exception being a strict vegan diet. People on vegan diets should take supplemental B12. Most healthy people who eat a varied diet do not need B12 supplements or B12 testing in absence of medical findings.

However, several medical conditions can affect the need for B12. Medications, especially metformin and proton-pump inhibitors like omeprazole, interfere with absorption, so more intake is needed to prevent deficiency. The DVs are not irrelevant, but one does need to take into account a person’s medical condition.

The most important medical issue is poor absorption due to a specific autoimmune disease: pernicious anemia. With this condition, the body destroys the cells that make a protein called “intrinsic factor,” without which the body cannot efficiently absorb B12. The 2.4 mcg of B12 that is fine for most people is totally inadequate when it comes to pernicious anemia. Low vitamin B12 can have neurological and psychiatric manifestations.

If your neurologist or oncologist was worried about pernicious anemia, they would recommend oral doses of 1,000-2,000 mcg daily, or they would prescribe B12 injections.

If you don’t have pernicious anemia, the 100 mcg in your multivitamin is more than you need, and 1,000 or 2,000 mcg is far more than you need. However, the harm from such high doses of B12 is essentially none as your body rapidly excretes the amount you don’t need.

DEAR DR. ROACH: My internist has discouraged me from getting the shingles vaccine, saying I “shouldn’t wake something that is sleeping.” I am in excellent health with an apparently strong immune system. Can you please comment on the likelihood of the vaccine triggering the virus versus the preventive effect of the vaccine? — N.T.

ANSWER: I strongly disagree with your internist. The new vaccine (Shingrix) is not a live vaccine, and large studies have confirmed that the vaccine does not trigger the reactivation of the varicella-zoster virus (which used to be a theoretical concern).

Two doses of the vaccine are more than 90% effective at preventing shingles, and new data suggest that there is an additional benefit in protecting people against heart attacks, strokes and dementia. I got my shingles vaccines as soon as I was eligible for them and recommend the vaccine to all my eligible patients.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu. (c) 2025 North America Syndicate Inc. All Rights Reserved