Vitamin B12: Normal Range, What High & Low Levels Mean

A Vitamin B12 blood test measures the amount of B12 — also called cobalamin — circulating in your bloodstream at the moment the sample is taken. B12 is a water-soluble vitamin your body relies on for three core jobs: making healthy red blood cells, synthesizing DNA, and keeping your nervous system working properly. Because your body can't manufacture B12 itself, the level in your blood reflects how much you've been absorbing from food or supplements over time. The test gives your doctor a snapshot of your current B12 status. Talk with your healthcare provider about what your specific result means for your situation.

For adults aged 18 and over, Mayo Clinic Laboratories defines the normal reference range for Vitamin B12 as 200 to 900 picograms per milliliter (pg/mL). A picogram is a tiny unit of measurement, which underscores just how little B12 the body needs — yet even small deviations matter. A result sitting comfortably between those two numbers generally suggests your circulating B12 is adequate. Keep in mind that reference ranges can vary slightly between laboratories depending on the method they use to run the test. Your lab report will show the range specific to where your sample was processed. Your healthcare provider is best placed to interpret your exact number in the context of your health history.

A B12 result above 900 pg/mL is considered elevated. The most straightforward explanation is that you've been taking B12 supplements or receiving B12 injections — in that case, a high reading is often expected and typically causes no symptoms on its own. However, unexpectedly high B12 levels not explained by supplementation can sometimes be linked to liver conditions such as hepatitis or cirrhosis, certain blood disorders like chronic myelogenous leukemia or polycythaemia vera, some solid tumours, or kidney disease. Because some of these causes are serious, an unexpectedly high result warrants a conversation with your healthcare provider to explore what might be driving the elevation.

Several things can push your B12 below 200 pg/mL. Since B12 is found almost exclusively in animal products, following a vegan or strict vegetarian diet is a common cause. The most frequent cause in older adults is pernicious anaemia — an autoimmune condition where the body destroys intrinsic factor, a protein you need to absorb B12 in your gut. Gastric surgery, atrophic gastritis, and malabsorption conditions like celiac disease or Crohn's disease can have a similar effect. Long-term use of metformin (a diabetes medication) or proton pump inhibitors (PPIs, used for acid reflux) can also impair absorption over time. Reduced stomach acid with older age is another factor. Your healthcare provider can help identify the underlying cause in your case.

Low B12 can produce a wide range of symptoms because the vitamin affects both your blood and your nervous system. You might notice persistent fatigue and weakness, numbness or tingling in your hands and feet, difficulty walking or keeping your balance, a sore and inflamed tongue, or mouth ulcers. Some people develop pale or slightly yellow skin, irritability, mood changes, memory problems, or blurred vision. In more significant deficiency, a condition called megaloblastic anaemia can develop, which may cause shortness of breath, headaches, and a rapid heartbeat. Symptoms often develop gradually and can be easy to overlook. If any of these sound familiar, bring them up with your healthcare provider alongside your B12 result.

How often you should retest depends entirely on your individual circumstances — your diet, any underlying conditions, medications you take, your age, and the reason your doctor ordered the test in the first place. Someone who has just started B12 supplementation, for example, may retest in a few months to confirm levels are rising. Someone managing a chronic condition affecting absorption might need more regular monitoring. There's no universal retesting schedule that fits everyone. Rather than going by a general rule, the best approach is to ask your healthcare provider when a follow-up test makes sense for you and what result they'd want to see at that point.