Why would someone have to get B12 shots?

December 6, 2025 •

3 min read

According to the National Institutes of Health, up to 15% of people in the United States may have a vitamin B12 deficiency, making supplementation a necessary intervention for many. A doctor might prescribe B12 shots when an individual cannot absorb this vital nutrient through their digestive system, which is required for healthy nerve function, DNA synthesis, and red blood cell formation.

Quick Summary

B12 shots are prescribed for individuals who cannot absorb sufficient vitamin B12 through their gut due to medical conditions, dietary restrictions, or surgery. The injections bypass the digestive system, effectively treating deficiencies that can lead to anemia, fatigue, and permanent neurological damage.

Key Points

Malabsorption is a primary cause: Conditions like pernicious anemia, Crohn's disease, and post-gastric surgery limit the body's ability to absorb B12 from food, necessitating shots.
Vegans are at risk: Since B12 is primarily found in animal products, those on strict plant-based diets may need injections if fortified foods or supplements are insufficient.
Shots ensure higher absorption: Injections deliver B12 directly into the muscle, offering a significantly higher absorption rate (55-97%) compared to oral supplements (around 1.3%).
Severe symptoms require injections: Advanced B12 deficiency can cause neurological problems, such as tingling, memory loss, and difficulty walking, which are most effectively treated with shots.
Diagnosis is clinical: A doctor's diagnosis, often based on blood tests, is required to determine the need for B12 shots, as the cause dictates the treatment frequency and duration.

Core Reasons for Needing B12 Shots

Many people can get sufficient vitamin B12 from a diet rich in animal products like meat, fish, and dairy. However, for others, the path to absorption is blocked, making injections the most direct and effective treatment. The primary reasons for needing B12 shots generally fall into two categories: malabsorption issues and specific dietary choices.

Malabsorption Conditions

The inability to properly absorb vitamin B12 from food is the most common reason for requiring injections. This can stem from various health problems affecting the stomach and intestines:

Pernicious Anemia: This autoimmune condition prevents the production of intrinsic factor, a protein made in the stomach that is essential for B12 absorption. Without intrinsic factor, dietary B12 cannot be absorbed, regardless of how much is consumed.
Atrophic Gastritis: Characterized by chronic inflammation of the stomach lining, this condition can lead to a decrease in stomach acid and intrinsic factor production, hindering B12 absorption. It is more common in older adults.
Gastrointestinal Surgery: Procedures like gastric bypass or the surgical removal of parts of the stomach or small intestine (especially the terminal ileum) can drastically reduce the areas where intrinsic factor is produced or where B12 is absorbed.
Digestive Diseases: Chronic conditions such as Crohn's disease and celiac disease can damage the lining of the small intestine, impairing its ability to absorb nutrients, including B12.
Bacterial Overgrowth or Tapeworm Infestation: An overgrowth of bacteria in the small intestine or an intestinal parasite like the fish tapeworm can compete for and consume the available B12, leaving the host deficient.

Dietary Deficiencies

Vitamin B12 is not naturally found in plant-based foods, making a strict vegan or vegetarian diet a significant risk factor for deficiency if not properly managed with supplements. While oral supplements are often recommended, in some severe cases or for those with absorption issues, injections may be needed initially to restore levels quickly.

Comparison: B12 Shots vs. Oral Supplements


Feature	B12 Shots (Intramuscular Injection)	Oral Supplements (Pills/Tablets)
Absorption	Very high, directly into the bloodstream (55-97%).	Variable and often low, relying on intrinsic factor (1.3% of 1000mcg dose).
Bypass GI Tract	Yes, completely bypasses the digestive system.	No, must be absorbed via the gastrointestinal tract.
Treatment Speed	Fast-acting; quickly reverses severe deficiencies.	Slower onset, though high doses can be effective over time.
Frequency	Less frequent, often monthly or every few months after initial treatment.	Daily dosage is often required for maintenance.
Medical Oversight	Requires prescription and administration by a healthcare provider.	Available over-the-counter; user manages dosage and frequency.
Primary Use	Treatment for severe deficiency, pernicious anemia, and malabsorption disorders.	Prevention and maintenance for dietary deficiencies or mild cases.

Symptoms of Vitamin B12 Deficiency

Untreated B12 deficiency can lead to serious and irreversible complications. It's crucial to recognize the symptoms that might necessitate a doctor's visit and blood test.

Common symptoms include:

Persistent fatigue and weakness
Numbness or tingling in the hands and feet (peripheral neuropathy)
A sore, smooth, or inflamed tongue
Balance and walking difficulties
Cognitive changes, such as confusion, memory loss, and poor concentration
Mood swings, irritability, or depression
Megaloblastic anemia, causing paleness, shortness of breath, and dizziness

The Role of B12 Injections in Treatment

For those with severe deficiency or malabsorption issues, B12 injections are the gold standard for treatment. The synthetic form of the vitamin, cyanocobalamin or hydroxocobalamin, is injected directly into a muscle. This bypasses any digestive absorption problems, delivering a high, therapeutic dose straight into the bloodstream. An initial course of frequent shots may be followed by a less frequent maintenance schedule, depending on the underlying cause. These injections effectively restore blood levels, reverse anemia, and can halt or slow the progression of neurological symptoms.

Conclusion

While many can maintain adequate vitamin B12 levels through diet, requiring B12 shots is a medical necessity for individuals with absorption disorders, pernicious anemia, or certain surgical histories. These injections offer a direct and highly effective route to replenish B12 stores, treat symptomatic deficiency, and prevent long-term neurological damage. For those at risk, regular medical consultation and blood monitoring are essential for timely diagnosis and proper treatment. It is always important to consult a healthcare professional to determine the appropriate treatment plan for a B12 deficiency.

Authority Link

For more in-depth information from the National Institutes of Health, visit their resource page on Vitamin B12.

Frequently Asked Questions

Pernicious anemia is the most common medical cause. It is an autoimmune condition where the body cannot produce intrinsic factor, a protein vital for absorbing vitamin B12 from the gut.

Yes, if an individual follows a strict vegan diet without proper supplementation, it can lead to a severe deficiency that may require initial injections to restore levels quickly, especially if absorption is also a concern.

Yes, B12 shots are only available by prescription from a healthcare provider after a clinical diagnosis of deficiency has been made through testing.

The frequency of injections varies depending on the underlying cause. Initially, a person may need shots every day or week, which then transitions to a monthly or bi-monthly maintenance schedule for chronic conditions.

B12 shots offer significantly higher absorption rates because they bypass the digestive system entirely. This makes them more effective for people with malabsorption issues and for quickly correcting a severe deficiency.

Severe deficiency can cause neurological symptoms such as nerve damage (neuropathy), memory loss, cognitive decline, irritability, and difficulties with walking and balance.

While the process is usually performed by a healthcare provider, in cases of long-term chronic conditions, some patients may be taught to self-administer the injections at home.