When people hit age 50, they often joke is that “it’s all downhill from here.” So it’s not surprising that someone who is tired all the time, or suffers bouts of constipation, for instance, might shrug these things off as a natural part of aging. But in fact, for anyone, especially people over 50, these symptoms could signal a vitamin B12 deficiency. According to the Centers for Disease Control and Prevention, 15 percent or more of older adults may be B12 deficient.

Other symptoms of B12 deficiency include not feeling hungry, numbness or tingling in your hands and feet, balance problems, confusion and poor memory.

Vitamin B12 is needed to make red blood cells and DNA. It also helps keep nerve cells healthy.

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Why you may be low in B12

Problems with B12 deficiency can crop up as people age because their bodies may not produce as much stomach acid as they did before. You need this acid to break down and absorb B12 from food, so a lack of it can mean a lack of B12. Being a vegan can also boost your risk of deficiency since some of the best sources of this vitamin are beef and poultry. People with celiac disease or Crohn’s disease and those who have had certain types of weight loss surgery may also have trouble absorbing enough B12 from food.

A simple blood test can reveal a B12 deficiency. In general, if your levels fall below 170 to 250 picograms per milliliter, you may be deficient, according to the National Institutes of Health (NIH).

Food, supplements and injections

If a blood test confirms a vitamin B12 deficiency, you have several options. If you’re only slightly deficient, your doctor may advise you to take a daily multivitamin (nearly all multivitamins contain B12). If your deficiency is more serious, a multivitamin won't do the trick; you’ll need either oral B12 supplements or B12 injections (usually given weekly at first). People with absorption problems or very low B12 levels may need injections.

B12 is also available as a prescription nasal spray (Nascobal). It's applied to one nostril once a week. People who want to avoid injections or who have trouble absorbing oral doses of B12 may get better results with the spray.

In general, the NIH advises people over 50 to get most of their B12 from supplements or from foods fortified with the vitamin. It’s easier for the body to absorb the B12 in these foods — which include low-fat milk and some grains and breakfast cereals — than from natural food sources.Check the label to see if B12 has been added and how much. Adults need 2.4 micrograms a day of B12 according to the Institute of Medicine.

If you decide to take a supplement on your own, a good rule of thumb is to stick to no more than 100 percent of the daily value unless your doctor tells you otherwise, advises Andrea Giancoli, MPH, RD, a nutrition consultant in Los Angeles.

Check your meds

Your medicine cabinet may be making it harder for you to keep a healthy B12 level. Drugs that can interfere with the absorption of the vitamin include proton pump inhibitors such as Prilosec (omeprazone) and Prevacid (lansoprazole), which are used to treat peptic ulcers and gastroesophageal reflux or GERD. Drugs known as H2 blockers, which include Pepcid (famotidine) and Zantac (ranitidine), pose similar problems, as can the diabetes drug metformin.

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If you use any of these medicines, ask your doctor for advice on keeping your B12 levels healthy.

Finally, if you take a lot of folic acid, this could hide a B12 deficiency. Before getting your B12 level checked, be sure to tell your doctor how much folic acid you are taking.

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.