What are the side effects of low vitamin K?

November 16, 2025 •

5 min read

While rare in healthy adults, vitamin K deficiency is a serious concern, especially for infants. Understanding the serious side effects of low vitamin K is critical for timely intervention, as this condition can lead to significant and dangerous health complications.

Quick Summary

Low vitamin K levels impair blood clotting, causing excessive bleeding, easy bruising, and poor bone health. The deficiency is rare in adults but a serious risk for newborns and those with malabsorption, requiring prompt medical intervention.

Key Points

Bleeding is Primary Concern: Impaired blood clotting is the main side effect of low vitamin K, leading to excessive bleeding and easy bruising.
Newborns are High-Risk: Infants are especially vulnerable to a severe form of bleeding called VKDB and are routinely given a vitamin K shot at birth to prevent this.
Bone Health is Affected: In addition to clotting, vitamin K is essential for bone mineralization, and deficiency can contribute to osteoporosis.
Malabsorption is a Major Cause: In adults, low vitamin K is often linked to underlying medical conditions like celiac disease, cystic fibrosis, or liver disorders that hinder fat absorption.
Medications Can Interfere: Certain drugs, including anticoagulants like warfarin and some antibiotics, can interfere with vitamin K's function and absorption.
Symptoms of Internal Bleeding: Severe deficiency can cause internal bleeding, with signs including bloody or tarry stools, blood in the urine, and, most dangerously, intracranial hemorrhage.
Treatment is Effective: The condition is treatable with vitamin K supplementation, administered either orally or by injection, depending on the cause and severity.

The Critical Role of Vitamin K

Vitamin K is a crucial fat-soluble vitamin essential for several vital bodily functions, most notably the production of blood-clotting proteins. The body needs vitamin K to create proteins, including prothrombin, that are necessary for blood coagulation. Without sufficient vitamin K, the blood's ability to clot is severely compromised, leading to the primary side effect of deficiency: uncontrolled bleeding. Beyond its function in clotting, vitamin K is also vital for bone health, helping to regulate calcium and promoting proper bone mineralization. There are two main forms of this vitamin: vitamin K1 (phylloquinone), found in plant-based foods like leafy green vegetables, and vitamin K2 (menaquinone), produced by intestinal bacteria.

Major Side Effects of Low Vitamin K

The side effects of low vitamin K are predominantly linked to the impaired function of blood-clotting proteins. While mild deficiencies may go unnoticed, more significant deficiencies can lead to severe and life-threatening issues.

Excessive Bleeding and Easy Bruising

One of the most noticeable side effects is the tendency to bleed excessively and bruise easily. This can manifest in several ways:

Easy bruising: Bruises may appear with little to no provocation due to blood leaking from compromised capillaries.
Bleeding from wounds: Even minor cuts or scrapes may bleed for a longer-than-normal duration.
Mucosal bleeding: This includes frequent nosebleeds (epistaxis) and bleeding from the gums.
Heavy menstrual periods: Women with low vitamin K may experience abnormally heavy menstrual bleeding (menorrhagia).
Subungal hemorrhages: Small, thin lines of blood may appear under the nails.

Life-Threatening Internal Hemorrhage

In severe cases, vitamin K deficiency can cause dangerous internal bleeding. This is a medical emergency that requires immediate treatment.

Gastrointestinal (GI) bleeding: Bleeding in the stomach or intestines can lead to dark, tarry stools (melena) or visible blood in the stool.
Intracranial bleeding: The most severe complication, especially in newborns, is bleeding within the skull or brain. This can cause brain damage, seizures, or death.

Impaired Bone Health

Vitamin K's role extends beyond clotting to the activation of proteins crucial for building and maintaining bone density, such as osteocalcin. Without sufficient vitamin K, these proteins cannot function correctly, potentially leading to:

Poor bone mineralization: The process of incorporating minerals into bone tissue is hindered.
Osteoporosis: A condition characterized by brittle and porous bones, leading to a higher risk of fractures.

Other Potential Symptoms

Less common but still important side effects can include fatigue and anemia, which may result from chronic blood loss. Newborns with a deficiency may also show signs like jaundice (yellowing of the skin and eyes), excessive sleepiness, or vomiting.

Special Risk: Low Vitamin K in Newborns

Vitamin K Deficiency Bleeding (VKDB) is a serious and potentially life-threatening condition that affects newborns. The risk is high for several reasons:

Limited placental transfer: Vitamin K does not transfer efficiently from the mother to the fetus during pregnancy.
Immature intestinal flora: Newborns' guts do not yet contain the bacteria necessary to produce their own vitamin K.
Low vitamin K in breast milk: Breast milk has a relatively low concentration of vitamin K compared to formula. For these reasons, a standard practice in many countries is to administer a single vitamin K injection to newborns shortly after birth to prevent VKDB.

Causes of Low Vitamin K in Adults

While dietary deficiency is rare in healthy adults, certain conditions and medications can cause low vitamin K levels.

Fat malabsorption disorders: As a fat-soluble vitamin, vitamin K requires dietary fat for proper absorption. Conditions like celiac disease, cystic fibrosis, and certain liver or gallbladder diseases that impede fat absorption can lead to deficiency.
Certain medications: Long-term use of certain medications, including antibiotics that disrupt the gut's bacterial flora and anticoagulants like warfarin, can interfere with vitamin K levels.
Severe malnutrition: Extremely poor diet lacking in vitamin K-rich foods can lead to deficiency.

Comparison of Symptoms: Low Vitamin K in Adults vs. Newborns


Symptom Category	Adults	Newborns
Bleeding & Bruising	Easy bruising, prolonged wound bleeding, mucosal bleeding (nose, gums), heavy menstrual periods.	Bleeding from umbilical cord or circumcision site, bruises on head/face, nosebleeds, petechiae (tiny red spots).
Internal Bleeding	Gastrointestinal bleeding (bloody or tarry stools), hematuria (blood in urine).	Gastrointestinal or intracranial hemorrhage, potentially leading to brain damage.
Bone Health	Reduced bone mineralization, increased risk of osteoporosis.	Poor bone development and mineralization.
Other Signs	Fatigue, weakness.	Jaundice (yellowing of skin/eyes), excessive sleepiness, seizures, vomiting.

Diagnosing and Treating Vitamin K Deficiency

Diagnosis of vitamin K deficiency typically involves a blood test called a prothrombin time (PT) test, which measures how long it takes for your blood to clot. A prolonged clotting time can indicate a deficiency.

Treatment is straightforward and highly effective. It involves supplementing with vitamin K, usually in the form of phytonadione (vitamin K1). The administration method and dosage vary depending on the severity of the deficiency and the patient's age. Oral supplements are common, but injections may be used if malabsorption is a concern or for more rapid correction. For adults on anticoagulants, a smaller, carefully monitored dose may be used.

Preventing a nutritional vitamin K deficiency in adults is generally simple and involves consuming a balanced diet rich in sources like leafy greens, vegetable oils, and certain fruits and fermented foods. For newborns, the standard preventative measure is the administration of a single vitamin K injection at birth.

Conclusion: Timely Action Prevents Serious Outcomes

Understanding the potential side effects of low vitamin K is crucial for preventing severe health problems. While deficiency is rare in healthy adults, specific risk factors like malabsorption disorders, liver disease, and certain medications warrant close monitoring. For newborns, the risk is significantly higher, which is why preventive vitamin K injections are so important. Symptoms such as easy bruising, excessive bleeding, and signs of internal bleeding should not be ignored. A timely diagnosis via a prothrombin time test and a prompt course of treatment with vitamin K supplements can effectively manage the condition and prevent life-threatening complications, especially those related to hemorrhage and bone health.

To learn more about the scientific details of this vitamin, you can visit the NIH Fact Sheet on Vitamin K.

Frequently Asked Questions

The main side effect is impaired blood clotting, which leads to excessive bleeding, prolonged wound healing, and easy bruising from even minor impacts.

Newborns are at the highest risk. In adults, those with fat malabsorption disorders (like celiac disease or cystic fibrosis), liver disease, or those taking certain medications (like warfarin or some antibiotics) are most vulnerable.

Treatment involves administering a vitamin K supplement, known as phytonadione. This can be given orally or via injection, with the dosage depending on the individual's age and the deficiency's severity.

Yes, vitamin K is essential for proper bone mineralization. A deficiency can lead to weakened bones and an increased risk of developing osteoporosis over time.

Signs can include dark, tarry stools (melena), bloody urine (hematuria), or vomiting blood. In severe cases, it can cause gastrointestinal or intracranial hemorrhage.

Most healthy adults get sufficient vitamin K from their diet, as it's common in green leafy vegetables. Additionally, intestinal bacteria produce some vitamin K, and the body can recycle its existing supply.

Severe vitamin K deficiency in pregnant women can be dangerous to the fetus and may cause bleeding and developmental anomalies. The growing fetus draws vitamin K from the mother's bloodstream.

A blood test called a prothrombin time (PT) test is primarily used. This test measures how long it takes for the blood to clot, and a prolonged time can indicate a vitamin K deficiency.

VKDB is a life-threatening bleeding disorder that affects newborns who have not received a vitamin K shot at birth. It is caused by the newborn's low vitamin K stores and inability to produce enough of their own in the first few weeks of life.