A normal vitamin B12 level generally falls between 200 and 950 pg/mL (picograms per milliliter). Values below 200 pg/mL are typically considered low, while the range between 200 and 300 pg/mL is a gray zone where deficiency symptoms can appear even though your result looks “normal” on paper. Up to 40% of people in Western populations fall into this low-to-marginal range.
Standard Reference Ranges
Most labs report a normal B12 range of roughly 160 to 950 pg/mL (118 to 701 pmol/L), though the exact cutoffs vary slightly between laboratories. The number that matters most is the low end. The NIH considers values below 200 to 250 pg/mL subnormal, and many clinicians use 200 pg/mL as the threshold below which deficiency is likely.
If your result is above 300 pg/mL, B12 deficiency is unlikely to be causing any symptoms. If it’s clearly below 200 pg/mL, further workup and treatment are usually straightforward. The tricky territory is everything in between.
The 200 to 300 pg/mL Gray Zone
A B12 level between 200 and 300 pg/mL is considered borderline. Your lab report may flag it as “normal,” but at this level your body may not have enough usable B12 to keep your nervous system and red blood cells running smoothly. This is surprisingly common, especially among people who eat little meat, fish, or dairy.
When your result falls in this range, a follow-up test for methylmalonic acid (MMA) can clarify whether your cells are actually short on B12. MMA is a substance that builds up when B12 is too low to do its job. A normal blood MMA level is usually under 0.40 µmol/L. If yours is elevated, it suggests a functional B12 deficiency even when your serum B12 looks borderline acceptable. Some providers also use a test called active B12 (holotranscobalamin), which measures the portion of B12 that’s actually available for your cells to use.
Symptoms of Low B12
B12 plays a central role in making red blood cells and maintaining your nervous system. When levels drop, the earliest signs are often vague: fatigue, brain fog, and difficulty concentrating. These overlap with dozens of other conditions, which is one reason B12 deficiency gets missed.
As a deficiency worsens, neurological symptoms become more specific and harder to ignore:
- Numbness or tingling in your hands and feet
- Balance problems or difficulty walking normally
- Memory trouble and confusion
- Vision changes
- Mood changes, including paranoia or depression in severe cases
On the blood cell side, B12 deficiency causes a type of anemia where red blood cells grow abnormally large and can’t carry oxygen efficiently. This shows up as pale skin, shortness of breath, and dizziness. Left untreated, severe deficiency can cause spinal cord damage, incontinence, and permanent nerve injury. The neurological damage can sometimes become irreversible, which is why catching low B12 early matters more than people realize.
Common Causes of Low Levels
Your body can’t make B12 on its own. It comes entirely from animal-based foods (meat, fish, eggs, dairy) or supplements. Vegans and strict vegetarians are at higher risk for deficiency simply because their diet doesn’t include natural sources.
But dietary intake is only half the equation. You also need to absorb B12 properly, and several conditions interfere with that process. Pernicious anemia, an autoimmune condition that attacks the stomach cells involved in B12 absorption, is one of the most common causes. Crohn’s disease, other inflammatory bowel conditions, and prior stomach or intestinal surgery can also reduce absorption significantly.
Medications are another major factor. Metformin, widely prescribed for type 2 diabetes, can lower B12 levels in up to 1 in 10 people who take it. The risk increases with higher doses and longer use. Proton pump inhibitors, the acid-reducing drugs many people take for reflux, also impair B12 absorption over time. Older adults absorb less B12 from food in general because stomach acid production naturally declines with age.
How B12 Deficiency Is Treated
Treatment depends on why your levels are low and how severe the deficiency is. For most people, oral supplements work well. Research from the American Academy of Family Physicians shows that taking 1,000 mcg of B12 by mouth daily raises levels just as effectively as getting the same dose by injection. At 2,000 mcg daily, oral supplements actually outperformed injections in head-to-head studies.
Injections are reserved for specific situations. If you have pernicious anemia, have had your stomach removed, or have a condition that severely limits intestinal absorption, you’ll likely need intramuscular B12 injections for life because pills simply won’t be absorbed reliably. For other types of malabsorption, oral doses of at least 1,000 mcg (1 mg) per day are recommended, since only a small fraction gets absorbed without the normal digestive pathway.
Most people notice improvements in energy and mental clarity within a few weeks of starting treatment. Nerve-related symptoms like tingling and numbness can take longer to resolve, sometimes months, and in cases of prolonged severe deficiency, some nerve damage may be permanent.
What High B12 Levels Mean
Levels above the normal range (roughly above 950 pg/mL) are less commonly discussed but worth understanding. In most cases, a high B12 result simply reflects recent supplementation or a diet rich in animal products. B12 is water-soluble, so your body excretes what it doesn’t need.
However, unexplained high B12 levels, especially in someone who isn’t supplementing, can occasionally signal liver disease, kidney problems, or certain blood disorders. The high reading happens because damaged liver cells release stored B12 into the bloodstream. If your level is elevated without an obvious dietary or supplement explanation, your provider may want to investigate further.
Who Should Get Tested
Routine B12 screening isn’t standard for everyone, but testing makes sense if you have symptoms of deficiency combined with at least one risk factor. The people who benefit most from proactive testing include vegans and vegetarians, adults over 65, anyone taking metformin or long-term acid reducers, people with Crohn’s disease or celiac disease, and those who’ve had gastrointestinal surgery. If you fall into one of these groups and notice persistent fatigue, tingling, or cognitive changes, a simple blood test can rule B12 in or out quickly.