Vitamin B12 Deficiency: Symptoms, Blood Tests, and What Your Levels Mean

Quick answer

Vitamin B12 supports your red blood cells and your nerves. It also supports your energy. Low B12 causes fatigue, brain fog, nerve pain, and a type of anemia. The standard range starts above 200 pg/mL. The healthy target is 500 to 900 pg/mL. A B12 supplement usually fixes it.

What B12 does for your body

Vitamin B12 is also called cobalamin. It has several jobs.

• It builds red blood cells. Low B12 can cause a type of anemia with unusually large cells.
• It supports your nerves. Low B12 can cause numbness, tingling, and balance trouble.
• It supports new cells. Your body needs it wherever cells divide quickly.
• It clears homocysteine, a marker linked to heart health.
• It supports energy production throughout your body.

Your symptoms usually match the system involved. Fatigue often comes from the anemia. Brain fog and mood changes often come from the nerve effects.

Why B12 deficiency happens

Not enough in your diet

B12 mainly comes from animal foods. These include meat, fish, eggs, and dairy. If you eat mostly plants, you need a supplement or fortified foods. Deficiency in this group is common. It is also fully preventable.

Trouble absorbing it

This is more common than a diet gap. Absorbing B12 needs stomach acid. It also needs a protein called intrinsic factor, and a healthy final section of your small intestine.

Several conditions get in the way.

• Atrophic gastritis. Common with age, it lowers stomach acid.
• Pernicious anemia. Your immune system attacks the cells that make intrinsic factor.
• H. pylori infection. This can block absorption too.
• Long-term acid reducer use. These medicines lower stomach acid.
• Long-term metformin use. This diabetes medicine can interfere with absorption.
• Gut conditions. Crohn's disease, celiac disease, and past stomach surgery all affect absorption.

Getting older

B12 absorption slowly declines with age. Adults over 50 can become deficient even while eating enough B12.

The blood tests

Serum B12

The standard range starts above 200 pg/mL. The healthy target is 500 to 900 pg/mL.

This is the usual test. But the wide standard range hides real problems. Many adults with B12 between 200 and 400 still have nerve symptoms and fatigue. Their result still reads as "normal." This is why the bottom of the B12 range is a problem rather than a pass.

Methylmalonic acid, or MMA

The standard range is under 0.4 µmol/L.

This is a more precise marker. MMA rises when your cells are truly short on B12. This can happen even if your serum B12 looks normal. MMA helps confirm a real deficiency.

Homocysteine

The standard range is under 15 µmol/L. The healthy target is under 8.

Homocysteine rises when B12 or folate runs low. It is a useful second check.

Holotranscobalamin

This measures active B12 more precisely. It is not offered everywhere, but it helps when standard results are unclear.

What your number means

The 200 to 500 zone matters most. People in this range can have real symptoms. Their lab report may still say "normal."

Symptoms of B12 deficiency

Nerve related

• Numbness or tingling in your hands and feet.
• Balance or walking trouble.
• Memory trouble and brain fog.
• Trouble concentrating.
• Mood changes, like low mood or irritability.

From anemia

• Ongoing tiredness.
• Pale skin.
• Shortness of breath with activity.
• A fast heartbeat.

Digestive

• A smooth, sore tongue.
• Diarrhea or constipation.
• Poor appetite.

Other

• Hair changes.
• Light headedness.

One important point. Nerve symptoms can appear and worsen without any anemia at all. This is why B12 alone, without a look at your symptoms, can miss real deficiency.

Common patterns

Pattern 1: Classic deficiency

• Serum B12 under 200 pg/mL.
• MCV over 100, a sign of large red blood cells.
• Often comes with nerve symptoms.
• What to do: start B12, and look for the cause.

Pattern 2: Deficiency despite a "normal" result

Doctors see this pattern the most today.

• Serum B12 between 200 and 400 pg/mL. This reads as "normal."
• MCV upper normal or slightly high.
• MMA and homocysteine often high, if tested.
• Symptoms are usually present.
• What to do: supplement, then retest at 12 weeks.

Pattern 3: Plant-based diet

• B12 falling slowly over years.
• Homocysteine often high.
• What to do: take a daily B12 supplement for life.

Pattern 4: Age-related absorption trouble

• B12 low or borderline. This is usually seen in adults over 60.
• May involve atrophic gastritis or pernicious anemia.
• What to do: try high-dose oral or under-the-tongue B12. Some cases need injections.

Pattern 5: Medicine-related

• B12 declining slowly. This happens on long-term metformin or acid reducers.
• What to do: check B12 yearly on these medicines, and supplement as needed.

How to fix B12 deficiency

Oral methylcobalamin

Take 1,000 µg daily. High-dose oral B12 works even with poor absorption. Enough of it gets through by sheer volume. Methylcobalamin, the active form, works better than cyanocobalamin for most people.

Under-the-tongue forms

These absorb through the mouth, bypassing the gut. They help with absorption problems.

Injections

Doctors reserve these for severe deficiency or nerve symptoms. Pernicious anemia and major absorption trouble also call for them.

Food sources

Animal foods supply B12. These include beef, pork, chicken, fish, eggs, and dairy. Fortified cereals and nutritional yeast help those who avoid animal foods.

Food alone rarely fixes an existing deficiency. Most people need a supplement first.

Retest at 12 weeks

This confirms the fix worked. For ongoing care, a maintenance dose of 500 µg daily works well for most adults.

Why borderline results get missed

The standard range starts above 200 pg/mL. It was set decades ago. It is wider than what current research supports. Nerve symptoms can show up at a level of 250 to 400 pg/mL. A standard report still reads that as "normal."

If you have symptoms with a borderline result, consider these steps.

1. Ask for MMA and homocysteine to check your true status.
2. Try a 12 week supplement trial. Feeling better is itself a useful clue.
3. Talk with a doctor familiar with these tighter, healthier targets.

How &you Labs reports B12

The standard &you Labs panel checks B12 alongside folate, ferritin, and vitamin D. MMA and homocysteine get added when your B12 sits in the borderline zone.

Your report reads B12 together with your cell size and folate level. The full picture matters more than any single number. If your energy has not improved despite past "normal" results, a &you Labs panel checks B12 against the healthy target, not just the wide standard range.

Frequently asked questions

How fast does B12 treatment work?

Your blood count often improves within 4 to 8 weeks. Nerve symptoms can take 3 to 6 months to improve, or longer. Damage from a long, untreated deficiency can sometimes be permanent.

Should I take B12 even if my level is "normal"?

Not usually. If your B12 sits in the healthy target of 500 to 900, and you have no symptoms, you likely do not need it. If your level is between 200 and 500 and you have symptoms, a supplement makes sense.

Are under-the-tongue B12 tablets better than regular pills?

Both forms work well for most people. Under-the-tongue forms help most with absorption problems. For severe deficiency or pernicious anemia, an injection works best.

Can B12 deficiency be reversed?

Yes, in most cases. Blood count changes and many nerve symptoms improve with treatment. Damage from a long delay can sometimes leave lasting effects.

My doctor says my B12 is "fine," but I still feel unwell. Why?

Your B12 may sit in the 200 to 400 zone. This is technically normal, but often not enough. You may have a deficiency that only MMA or homocysteine can catch. Or something else may explain your symptoms. Ask about the healthy target range, and consider further testing.

Key takeaways

• Low B12 causes fatigue, nerve symptoms, and a type of anemia with large red blood cells.
• The healthy target is 500 to 900 pg/mL. The standard range starts at 200, and it misses many people with real symptoms.
• MMA and homocysteine confirm a true deficiency when your B12 sits in the borderline zone.
• Common causes include a plant-based diet, aging, pernicious anemia, and long-term metformin use.
• A daily B12 supplement usually fixes it. Severe cases may need injections.

Ready to check your B12 against a healthier target? Book your &you Labs panel. Your results come back with both the standard range and the healthy target range, plus a doctor's written interpretation.