Ask the Doctors - Are large doses of B12 safe?

Dr. Robert Ashley, MD

Dear Doctor: I’m almost 90 years old and always tired. I took two capsules of 2,000 mcg vitamin B12 the other night, and felt stronger and not as tired. Is it safe?

My first thought is that you might have B12 deficiency, caused either by inadequate intake or by inadequate absorption of B12. Animal products such as meat, dairy and eggs are the only sources of B12 in humans, and the recommended daily allowance is 2.4 micrograms per day. The average intake of an individual in the United States is approximately 3.4 micrograms per day, but if you follow a vegan diet, you obviously lack sources of B12. An evolutionarily favorable fact is that one-half of your B12 is stored in your liver, and it takes many years for these stores to be depleted.

If your B12 intake is sufficient, it may be your ability to absorb B12 that is compromised. This can occur if you have inflammation of your stomach lining or disorders of the small intestine, such as Crohn's disease. Antacids and proton pump inhibitors such as Prilosec, Protonix, Prevacid, Aciphex, Zantac and Pepcid also can lead to B12 deficiency, because they reduce the stomach acid necessary to release the B12 from food. Metformin, a drug used to treat type 2 diabetes, can also decrease absorption of B12 in the small intestine of 10 percent to 30 percent of patients.

Lastly, age can be a risk factor for B12 deficiency. Multiple studies in this and in other countries have found B12 deficiency in 10 percent to 24 percent of people 75 and older. The causes may be both inadequate intake and inadequate absorption. Significant B12 deficiency can lead to anemia, nerve damage and dementia.

But let us say that your B12 levels are normal. Is there any benefit to taking more B12, and is there any danger in doing so? Many of my patients, who are not B12 deficient, feel greater energy and improvement of their health with B12 injections. I am not certain why this happens, and it doesn't appear to have a lasting effect. It is possible that extra amounts of B12 can improve the metabolic function and allow for the B12 to be utilized better.

Water soluble vitamins, like B12, can be tolerated at higher doses without adverse effects upon the body, and the National Academy of Medicine has not established an upper limit of Vitamin B12 intake. The dose of 2000 micrograms of B12 is more than 800 times the recommended daily amount so, on the face of it, this dose seems too high, but as you increase the amount of B12 in your diet, less of a percentage will be absorbed.

Before continuing the B12, I would recommend getting your B12 level tested with your doctor. If you show a deficiency, your doctor may want to investigate why you are deficient – if you are – and to gauge how much B12 you might need.

If you are not deficient, I believe it is safe to take a B12 supplement at a lower dose; you don't even have to take it every day because B12 is well-stored by the body. Further, if you want to ensure better absorption of B12, you can try preparations that can be absorbed under the tongue. The important thing is to investigate why this B12 had such an impact – and how you can safely replicate the effects.

Robert Ashley, MD, is an internist and assistant professor of medicine at the University of California, Los Angeles.

Ask the Doctors is a syndicated column first published by UExpress syndicate.