Why Would You Be Prescribed B12 by a Doctor?

December 18, 2025 •

3 min read

Approximately 6% of people under 60 in the U.S. and UK may have a B12 deficiency, with the prevalence rising to 20% in those over 60, often requiring a doctor's prescription. Prescribed B12 is typically for individuals whose bodies cannot absorb or produce enough of this essential vitamin from food alone, or for specific medical conditions.

Quick Summary

Prescribed B12 is needed for deficiencies caused by dietary issues, medical conditions, medications, or surgical procedures. Malabsorption problems like pernicious anemia or Crohn's disease are common causes requiring supplementation. It addresses symptoms such as fatigue, anemia, nerve damage, and cognitive issues.

Key Points

Malabsorption Issues: The most common reason for a B12 prescription is an inability to properly absorb the vitamin, often due to conditions like pernicious anemia, Crohn's, or celiac disease.
Autoimmune Attack: Pernicious anemia is an autoimmune disorder that attacks intrinsic factor, a protein vital for B12 absorption, requiring lifelong supplementation.
Surgical Impact: Individuals who have undergone bariatric or stomach surgery may require prescribed B12 because the procedure alters the digestive tract and impairs absorption.
Medication Side Effects: Long-term use of certain medications, including metformin for diabetes and acid-reducing drugs, can cause B12 deficiency.
Dietary Insufficiency: Strict vegans and vegetarians are at risk of deficiency due to limited B12 sources and may require prescribed supplements if dietary fortification is insufficient.
Neurological Symptoms: Untreated B12 deficiency can lead to irreversible neurological damage, making prescribed treatment crucial for addressing symptoms like numbness, memory loss, and balance problems.

Vitamin B12, also known as cobalamin, is a vital nutrient necessary for red blood cell production, neurological function, and DNA synthesis. While most people get sufficient B12 from their diet, certain factors can lead to a deficiency serious enough to require a doctor's prescription. These prescribed treatments, which can include injections or high-dose oral supplements, are reserved for cases where dietary changes are insufficient.

Medical Conditions That Cause B12 Deficiency

Many underlying health problems interfere with the body's ability to absorb vitamin B12, making prescription-strength supplementation necessary. A doctor will typically perform blood tests to confirm the deficiency and identify the root cause.

Pernicious Anemia

Pernicious anemia is a common reason for a B12 prescription. It's an autoimmune condition where the body attacks parietal cells in the stomach that produce intrinsic factor, a protein vital for B12 absorption in the intestines. This prevents B12 from food being properly used, leading to deficiency. People with this condition often need regular B12 injections for life.

Gastrointestinal Disorders

Diseases affecting the digestive tract can impair B12 absorption.

Crohn's Disease and Celiac Disease: These conditions can damage the small intestine where B12 is absorbed.
Atrophic Gastritis: Chronic stomach lining inflammation can reduce intrinsic factor and stomach acid production needed for B12 release.
Small Intestine Bacterial Overgrowth (SIBO): Bacteria in the small intestine can consume B12, reducing absorption.

Surgical Procedures Affecting B12 Absorption

Surgeries altering the digestive system significantly increase deficiency risk and often require prescribed supplementation.

Gastrectomy: Removal of the stomach eliminates the source of intrinsic factor.
Bariatric Surgery: Procedures like gastric bypass can bypass parts of the digestive system where B12 absorption occurs.

Lifestyle and Diet-Related Causes

Some dietary choices can lead to deficiencies needing medical intervention.

Vegan or Vegetarian Diets: B12 is mainly in animal products. Strict vegans and some vegetarians not using fortified foods or supplements are at high risk.
Chronic Alcohol Use: Excessive alcohol can damage the digestive system and impair B12 absorption.

Medication-Induced Deficiencies

Certain long-term medications can interfere with B12.

Metformin: This diabetes drug can lower B12 levels over time.
Proton Pump Inhibitors (PPIs) and H2 Blockers: Acid reflux medications reduce stomach acid needed for B12 absorption from food.

Comparing B12 Treatment Options


Feature	Intramuscular Injections	High-Dose Oral Supplements
Application	Injected directly into muscle by a healthcare professional (or self-administered).	Taken as a pill, capsule, or sublingual tablet.
Absorption Mechanism	Bypasses the gastrointestinal tract entirely, making it ideal for malabsorption issues like pernicious anemia.	Relies on a small amount of passive absorption, independent of intrinsic factor.
Speed of Action	Considered the fastest way to replenish depleted B12 stores, especially in severe deficiencies.	Effective but may take longer to correct severe deficiencies due to passive absorption.
Convenience	Requires periodic visits to a clinic or self-injection, which can be less convenient.	Offers greater convenience, avoiding the need for injections.
Cost	Can sometimes be covered by insurance, but may involve higher costs per dose.	Often more affordable, especially for long-term maintenance.

Symptoms of B12 Deficiency

Recognizing symptoms is vital for preventing irreversible neurological damage. Common signs include:

Persistent fatigue and lethargy.
Numbness or tingling in hands and feet.
A sore, red, and swollen tongue (glossitis).
Anemia, causing paleness, dizziness, and shortness of breath.
Cognitive issues like memory loss or confusion.
Psychological problems such as depression or mood swings.
Balance and coordination problems (ataxia).

Conclusion

Being prescribed B12 is typically due to an underlying issue preventing sufficient intake or absorption through diet. Conditions like pernicious anemia, gastrointestinal disorders, surgery, or certain medications can necessitate a prescription to ensure adequate levels for red blood cell production and nerve function. Early diagnosis and treatment are crucial to manage symptoms and avoid long-term neurological complications. Always consult a healthcare provider for diagnosis before supplementation.

For more detailed information on pernicious anemia and its treatment, consult authoritative medical resources. National Institutes of Health (NIH) | Pernicious Anemia

Frequently Asked Questions

A doctor primarily prescribes B12 to treat a confirmed vitamin B12 deficiency, which is often caused by an underlying medical condition, malabsorption issue, or a restricted diet.

Pernicious anemia is an autoimmune disease where the body cannot produce intrinsic factor, a protein needed to absorb B12 from food. It is treated with regular B12 injections to bypass the digestive tract and ensure the vitamin reaches the bloodstream.

Yes, some medications can interfere with B12 absorption. For example, long-term use of metformin (for diabetes) and proton pump inhibitors (for acid reflux) have been shown to lower B12 levels.

B12 injections are typically used for severe deficiencies or for patients with malabsorption issues, as they bypass the digestive system entirely and ensure maximum absorption. They are often the standard of care for conditions like pernicious anemia.

Symptoms that may suggest a need for B12 testing include persistent fatigue, muscle weakness, a sore tongue, memory problems, numbness or tingling in the hands and feet, and mood changes like depression.

Not always, but they are at a higher risk of deficiency because B12 is primarily found in animal products. A doctor may prescribe B12 if dietary sources, such as fortified foods, are not sufficient to maintain adequate levels.

If left untreated for an extended period, severe B12 deficiency can lead to permanent neurological damage, affecting the nervous system and brain. Early diagnosis and treatment are therefore essential.