B-12 comes in pill form and a new nasal spray, but injections are the fastest way to correct a severedeficiency, since it does not need to be absorbed in the digestive tract. Many patients learn to inject themselves or have a spouse do it to cut down on doctor visits. 'I have world travelers who take their B-12 and needles with them,' says Dr. Pocinki. He says he has seen low-grade depression lift when patients were given B-12 supplements.

Researchers are still investigating possible links between long-term B-12 deficiency and other health problems. It's known, for example, that B-12 and the other B vitamins help reduce homocysteine, an amino acid in the bloodstream that may be associated with heart disease. But several large prospective studies have not shown that taking B-12 supplements reduces the risk.

It's also not clear whether low B-12 raises the risk of dementia and Alzheimer's disease, although cognitive impairment brought on by low B-12 is sometimes mistaken for Alzheimer's.

'It's really easy to miss, and really easy to fix,' says Dr. Pocinki.

An additional concern: A high level of folic acid can mask symptoms of B-12 deficiency, and many people are getting more folic acid than they realize. Since 1996, all enriched flour must contain folic acid to reduce the risk of neural-tube defects, a severe malformation that occurs in developing fetuses. Some studies suggest that having high folic acid can make cognitive damage from low B-12 even worse.

'To this day, we are concerned about the trade-off,' says Dr. Rosenberg, who was on the advisory panel that recommended enriching flour.