Why do newborns get a vitamin K shot at birth?

Babies are born with very low vitamin K reserves, breast milk contains relatively little, and their gut bacteria (which produce some vitamin K in adults) haven't yet established. Without the routine injection, infants face a real risk of vitamin K deficiency bleeding, which can be serious. It's one of the most well-established preventive interventions in newborn care.

Does vitamin K deficiency in adults mean I'll bleed spontaneously?

Usually not spontaneously — it typically shows as bleeding that's excessive relative to the injury: cuts that bleed longer than expected, bruises from minor bumps, or heavier-than-normal menstrual periods. Spontaneous, unprovoked bleeding is less common and warrants urgent medical attention.

Can antibiotics cause vitamin K deficiency?

Yes, particularly broad-spectrum or long-course antibiotics. Gut bacteria produce a meaningful share of the body's vitamin K, and antibiotics that disrupt that bacterial population can reduce vitamin K availability, especially in people already eating few vitamin-K-rich vegetables.

Why do people on blood thinners need to watch their vitamin K intake?

Warfarin works by blocking vitamin K's role in clotting, so sudden large changes in vitamin K intake (a big increase from eating more leafy greens, or a drop from eating less) can throw off the drug's effect. The goal for people on warfarin isn't avoiding vitamin K, but keeping intake consistent week to week.

Vitamin K Deficiency Test: Check Your Symptoms Online

Based on NIH Office of Dietary Supplements guidance · Last reviewed: July 15, 2026 · How we source our content

Vitamin K's entire job is helping your blood clot properly, so deficiency has one of the more distinctive symptom pictures of any nutrient we cover — it's almost always about bleeding and bruising, rarely about fatigue or mood. That specificity makes it easier to recognize, but also easier to dismiss as "just how my body is" when it's actually a fixable pattern.

Easy bruising, heavy periods, slow-stopping bleeding?

Our free assessment weighs your symptoms and risk factors in about 2 minutes.

What low vitamin K looks like

Bruising more easily than seems proportionate to the bump that caused it
Cuts or nosebleeds that take longer than usual to stop
Heavier or longer menstrual periods
Blood in urine or stool — always worth a prompt medical check regardless of suspected cause
Excessive bleeding after surgery or dental work

Because vitamin K deficiency can overlap with other bleeding disorders and with certain medications, unexplained or worsening bleeding symptoms deserve medical evaluation rather than self-diagnosis — this is one area where a screening quiz is a starting point, not a substitute.

Who's actually at risk

Long-term antibiotic use

Gut bacteria produce a meaningful share of vitamin K; antibiotics that disrupt that bacterial population reduce availability.

Fat-malabsorption conditions

Vitamin K is fat-soluble — Crohn's disease, celiac disease and bile duct disorders all impair absorption.

Very low intake of leafy greens

Vitamin K is concentrated in leafy vegetables specifically — diets that rarely include them run lower intake than most people assume.

Newborns

Low birth reserves and undeveloped gut bacteria make infants a distinct, well-documented risk group — addressed by the routine vitamin K injection at birth.

Where our test fits

Our free assessment weighs bruising and bleeding symptoms against your medication history, diet, and any digestive conditions — because vitamin K deficiency in adults is almost always secondary to one of those, not a standalone dietary gap. It also flags when symptoms sound like they warrant medical attention beyond what a screening tool can offer, particularly for anyone on blood-thinning medication.

If you are low

Leafy greens — kale, spinach, collard greens, broccoli — are by far the richest sources, alongside Brussels sprouts and some vegetable oils. There's no established RDA in the traditional sense; the Adequate Intake is 120 mcg/day (men) or 90 mcg/day (women). If you're on warfarin or another vitamin K-dependent blood thinner, the goal isn't avoiding vitamin K — it's keeping intake consistent week to week, since sudden swings (in either direction) can affect how the medication works.

Medical disclaimer: This page and our assessment are educational tools, not medical advice or diagnosis. Unexplained bleeding, blood in urine or stool, or heavy menstrual bleeding deserve prompt medical evaluation regardless of what any online tool suggests.

Frequently asked questions

Babies are born with very low vitamin K reserves, breast milk contains relatively little, and gut bacteria haven't established yet. Without the routine injection, infants face a real risk of vitamin K deficiency bleeding — one of the most well-established preventive interventions in newborn care.

Usually not spontaneously — it typically shows as bleeding excessive relative to the injury: slow-stopping cuts, bruises from minor bumps, or heavier menstrual periods. Spontaneous, unprovoked bleeding is less common and warrants urgent medical attention.

Yes, particularly broad-spectrum or long-course antibiotics. Gut bacteria produce a meaningful share of the body's vitamin K, and disrupting that bacterial population reduces availability, especially alongside low intake of vitamin-K-rich vegetables.

Warfarin works by blocking vitamin K's role in clotting, so sudden large intake changes can throw off the drug's effect. The goal isn't avoiding vitamin K — it's keeping intake consistent week to week.

Find out in 2 minutes

Free vitamin K risk check — no email, no payment, no blood draw.