How long does B12 stay in the bloodstream?

December 22, 2025 •

3 min read

The biological half-life of vitamin B12 in the plasma is approximately 6 days, but this is only part of the story. Understanding how long does B12 stay in the bloodstream requires looking at its transport, storage, and the various factors that influence its levels in the body.

Quick Summary

Vitamin B12 circulates in the blood for a short period, with a half-life of about 6 days, while the liver stores reserves for years. Its presence in the bloodstream is influenced by factors like the absorption method, dosage, and a person's underlying health status.

Key Points

Blood Circulation is Short-Lived: The active form of B12 (HoloTC) has a bloodstream half-life of only about 1.5 hours, though other forms circulate longer.
Liver Storage is Long-Term: A significant amount of B12 is stored in the liver, with reserves lasting up to 5 years, which is why a deficiency takes time to develop.
Injections are Rapid and Direct: B12 injections provide a quick and potent boost by bypassing the digestive system, leading to high initial blood levels and rapid cellular uptake.
Oral Supplements Require Intrinsic Factor: Absorption of B12 from food and oral supplements depends on intrinsic factor, a protein produced in the stomach.
Deficiency Takes Time to Appear: Due to the liver's large storage capacity, it can take years for signs of a B12 deficiency to become apparent after intake stops or absorption issues begin.
Excess is Excreted: As a water-soluble vitamin, any excess B12 that saturates the body's binding proteins is efficiently excreted through the urine.

Understanding B12 Circulation vs. Storage

For many, the idea of a water-soluble vitamin suggests it is quickly excreted. While excess vitamin B12 is indeed eliminated through urine, its overall presence in the body is more complex due to sophisticated transport and storage mechanisms. The short period it remains actively circulating in the blood is distinct from its long-term storage, which primarily occurs in the liver and can last several years. This is why deficiencies can take years to develop, even when dietary intake is insufficient.

The Role of Transport Proteins

In the bloodstream, vitamin B12 does not travel freely. It is bound to specific carrier proteins, primarily transcobalamin (TC) and haptocorrin (HC), which dictate its function and movement.

Transcobalamin II (HoloTC)

This is the biologically active form of vitamin B12. Holotranscobalamin (HoloTC) binds to cell receptors throughout the body, allowing for cellular uptake and metabolism. It has a very short half-life in the bloodstream—approximately 1.5 hours—before it is delivered to cells. This active transport is crucial for immediate cellular needs.

Haptocorrin (HoloHC)

Most circulating B12 is bound to haptocorrin, which has a longer half-life of 9 to 10 days. This form is generally considered inactive and primarily functions as a transport and storage mechanism, especially for delivery to the liver where it is retained for long-term use.

B12 Metabolism and Elimination

The body is highly efficient at recycling B12 through a process known as enterohepatic circulation. B12 is secreted into the bile, and much of it is reabsorbed in the ileum. Only a small amount of B12 is lost through feces daily. However, when high doses are administered, especially via injections, the binding capacity of transport proteins is saturated, and the excess unbound B12 is rapidly eliminated by the kidneys in the urine within hours.

B12 Injection vs. Oral Supplementation

How B12 is delivered to the body has a significant impact on its blood levels.


Feature	B12 Injections	Oral Supplements
Absorption Method	Delivered directly into the muscle, bypassing the digestive system for rapid absorption.	Absorbed through the gastrointestinal tract, requiring intrinsic factor for efficient absorption.
Absorption Rate	Very rapid, with some excess excreted within hours.	Slower and limited by the capacity of the intrinsic factor mechanism.
Initial Blood Peak	A large, immediate increase in blood B12 levels is observed within 24-72 hours.	A more gradual and sustained increase over days or weeks, depending on the dose.
Typical Duration of Effect	Weeks to months, depending on the severity of deficiency and individual metabolism.	Requires consistent daily or weekly intake to maintain stable levels, especially for those with absorption issues.
Who Benefits Most	Individuals with severe deficiency, pernicious anemia, or malabsorption disorders.	Individuals with mild deficiencies or those on a diet low in B12 who can absorb it effectively.

Factors Influencing B12 Blood Levels and Storage

Beyond the method of administration, several factors determine how B12 behaves in the body:

Intrinsic Factor Production: A protein produced in the stomach, intrinsic factor is essential for absorbing B12 from food and oral supplements. Conditions like pernicious anemia prevent its production, leading to deficiency.
Diet: The primary source of B12 is animal products. Vegans and vegetarians are at higher risk of deficiency if they don't use fortified foods or supplements.
Digestive Health: Diseases such as Crohn's disease, celiac disease, and gastritis can impair B12 absorption. Surgical removal of parts of the stomach or small intestine also impacts absorption.
Medications: Some medications, including metformin and proton pump inhibitors, can interfere with B12 absorption over time.
Age: As people age, stomach acid production decreases, making it more difficult to absorb B12 from food sources.
Liver Health: Since the liver is the main storage site, liver disease can affect B12 storage and function.

Conclusion

In summary, the question of how long does B12 stay in the bloodstream is complex, with a distinction between its short-term activity and long-term storage. While active B12 circulates for only a few hours, the body recycles and stores large reserves for up to several years. The overall duration is significantly impacted by the method of delivery, a person's digestive health, and other lifestyle factors. For effective management of B12 levels, it's essential to consider these individual factors rather than focusing solely on a single blood-level reading. For more detailed information on vitamin B12, refer to the NIH Office of Dietary Supplements.

Frequently Asked Questions

The biological half-life of vitamin B12 in plasma is approximately 6 days. However, the active form of B12, bound to transcobalamin II, has a much shorter half-life of around 1.5 hours before it is delivered to cells.

The body stores a significant amount of vitamin B12, primarily in the liver, for a much longer period than it circulates in the blood. These stores can last anywhere from 3 to 5 years before becoming exhausted.

B12 injections cause an immediate and large spike in blood levels by bypassing the digestive tract. While some effects may be felt for weeks, oral supplements must be taken regularly to maintain a consistent level, especially for those with absorption issues.

Because the liver holds large reserves, a B12 deficiency caused by low intake or absorption issues typically develops slowly over years. Therefore, blood levels do not drop suddenly unless there is a severe underlying condition.

Factors include diet, the health of the digestive system, the presence of specific transport proteins (transcobalamin and haptocorrin), liver function, age, and certain medications.

Many people have B12 deficiency due to a lack of intrinsic factor, a protein needed for absorption, or other malabsorption disorders rather than inadequate dietary intake. These issues prevent the body from absorbing B12 effectively.

Yes. When high doses of B12 are taken, some can be absorbed via passive diffusion, bypassing the intrinsic factor pathway. This can temporarily elevate blood levels, but the excess is eventually excreted by the kidneys.