Vitamin B12: Testing, Deficiency, and When Supplementation Helps

Why B12 is easy to run low on

B12 from food is bound to protein and must be released by stomach acid and digestive enzymes, then carried by intrinsic factor made in the stomach and absorbed in the terminal ileum. Any step in that chain can fail: low stomach acid, acid-suppressing medications, atrophic or autoimmune gastritis, celiac disease, Crohn's disease, or prior bariatric or bowel surgery.

Dietary insufficiency takes years to produce deficiency because liver stores are large, but it does eventually produce it in people who eat no animal products. Medication-related and absorption-related causes are more common in midlife: metformin and proton pump inhibitors are both associated with lower B12 status.

How deficiency is actually diagnosed

The initial test is a total serum B12 level. In the AAFP's 2025 review of the topic, a level below 180 pg/mL is considered diagnostic of deficiency, while borderline values between 180 and 350 pg/mL warrant a methylmalonic acid (MMA) measurement, which is diagnostic when elevated. MMA rises because B12 is required to convert methylmalonyl-CoA; it is a functional marker rather than a measure of how much vitamin is circulating.

Homocysteine is sometimes measured as well, but it rises in several deficiencies and conditions, so it is less specific. The National Institute for Health and Care Excellence evidence review supporting its B12 guideline notes that no single test is a gold standard, which is why testing is interpreted alongside the clinical picture rather than in isolation.

Symptoms that should prompt testing

• —Fatigue, breathlessness on exertion, or a blood count showing large red cells (macrocytosis) — though neurological problems can occur with a normal blood count.
• —Numbness, tingling, or unsteadiness in the feet, and difficulty with balance or position sense.
• —Cognitive changes, low mood, or a sore, smooth tongue.
• —Any of these in a person taking metformin or a daily acid reducer, following a vegan or vegetarian diet, over age 60, or with celiac, Crohn's, or bariatric surgery history.

Oral versus injection

This surprises many patients: the AAFP review states that oral vitamin B12 supplementation can be used in most patients and is noninferior to intramuscular supplementation, with intramuscular administration reserved for severe deficiency or neurological manifestations. High-dose oral B12 is absorbed in small amounts by passive diffusion, which works even when the intrinsic-factor pathway is impaired.

That means a B12 injection is best understood as a reasonable option when a patient prefers it, has documented severe or neurological deficiency, or wants a supervised in-office treatment — not as a superior form of repletion for routine deficiency. It also means an injection given before testing can be a poor decision, because it raises the level without explaining why it was low.

One further caution from the same review: persistently high B12 levels above 1,000 pg/mL on two measurements have been associated with solid tumors, hematologic malignancy, and increased cardiovascular death. A high result is not automatically a reason to celebrate — it is a reason to be evaluated.

What Vitality Modern Medicine offers

In clinic, a B12 booster injection is available at $15, and it is one of the boosters included in the testosterone therapy program. It is offered after a brief health screening and a conversation about why you want it.

If fatigue is the reason, the online hormone-care pathway evaluates the common causes first: labs are ordered and completed, results are reviewed by your provider, and a plan follows the consultation rather than preceding it.

Prices shown are current clinic prices as of October 2026, are cash-pay, and can change. A provider decides whether a specific injection or supplement is appropriate for you before anything is administered.

This article summarizes published research for general education. It is not medical advice, does not recommend a specific product or dose, and does not replace evaluation by your own provider.