K3, also called menadione, is a synthetic form of vitamin K that the body can convert into active vitamin K2. Unlike the natural forms (K1 from leafy greens and K2 from fermented foods), K3 is made in a lab and lacks the chemical side chain that makes natural vitamin K immediately usable. Once inside the body, cells attach that missing side chain through a process called prenylation, turning K3 into the active form MK-4, a type of vitamin K2. That conversion is what gives K3 its biological function, but it also comes with significant toxicity risks that have led to its ban in human supplements in the United States.
How K3 Works in the Body
All forms of vitamin K serve as helpers for activating specific proteins. These proteins are essential for normal blood clotting, bone strength, and keeping calcium out of your arteries. K3 can fulfill these roles, but only after your body transforms it first.
The conversion happens in two steps. First, cells reduce K3 to a slightly different chemical state called the hydroquinone form. Then an enzyme called UBIAD1, found in tissues throughout the body, attaches a long carbon chain to it. The final product is MK-4, one of the most important forms of vitamin K2. Research published in The Journal of Biological Chemistry confirmed that only the reduced (hydroquinone) form of K3 can serve as the raw material for this conversion, not the original form you’d swallow.
This is actually the same pathway your body uses when it converts dietary vitamin K1 into K2. K1 from food gets broken down in the intestine into K3 as an intermediate step, and then tissues rebuild it into MK-4. So K3 is a natural middleman in vitamin K metabolism. The problem is that flooding the body with large amounts of this intermediate directly, rather than letting it form gradually from K1, overwhelms protective systems.
Why K3 Is Toxic to Humans
K3 interferes with glutathione, one of the body’s most important antioxidant defenses. Without adequate glutathione protection, cells become vulnerable to oxidative damage. The liver takes the hardest hit because it’s where K3 is primarily processed. Exposure increases oxygen uptake in liver cells, triggering a chain reaction of fat oxidation that damages and kills those cells.
The specific dangers include hemolytic anemia (destruction of red blood cells), jaundice, elevated bilirubin levels, and direct liver cell death. In infants, K3 toxicity can cause kernicterus, a form of brain damage from bilirubin buildup. These risks are serious enough that K3 has been banned from over-the-counter sale in the United States for human use. The natural forms, K1 and K2, have no established upper toxicity limit even at high doses.
K3 in Animal Feed
Despite being off-limits for human supplements, K3 remains widely used in animal nutrition. If you’ve read pet food labels, you may have seen “menadione sodium bisulfite” listed as an ingredient. This is a water-soluble form of K3 added to feed for dogs, cats, poultry, and livestock to prevent vitamin K deficiency.
The European Food Safety Authority reviewed K3’s use in animal feed and concluded that standard inclusion levels are safe for all animal species and pose no safety concerns for consumers eating meat, eggs, or dairy from those animals. Acute toxicity in animals only occurs at doses exceeding their actual nutritional requirements by a factor of at least 1,000. At the small amounts added to commercial feed, K3 is considered an effective and economical source of vitamin K activity. Still, some pet owners prefer foods using natural vitamin K sources, and premium pet food brands sometimes market the absence of menadione as a selling point.
How K3 Compares to K1 and K2
The three forms of vitamin K differ in where they come from, how well they’re absorbed, and what they do best. K1 (phylloquinone) comes from green vegetables and is preferentially retained in the liver, where it supports blood clotting. K2 (menaquinones) comes from fermented foods and animal products, circulates in the blood longer, and reaches tissues beyond the liver like bone and blood vessel walls. Among K2 subtypes, MK-7 is absorbed most efficiently, with blood levels reaching 10 times higher than K1 after a comparable dose.
K3 has no dietary source because it’s entirely synthetic. Its advantage in animal agriculture is cost and stability: it’s cheap to produce and easy to add to feed. But for humans, K1 and K2 provide the same biological benefits without toxicity concerns. K2 in the MK-7 form has a particularly long half-life in circulation, meaning it stays active in your blood longer and reaches more tissues. K1, while less bioavailable than K2 overall, is abundant in affordable foods like spinach, kale, and broccoli.
Experimental Use in Cancer Research
One area where K3 has drawn scientific interest is oncology. When combined with vitamin C in a 1:100 ratio, K3 showed a synergistic effect against human tumor cells in laboratory studies, inhibiting growth and triggering cell death at concentrations 10 to 50 times lower than either vitamin alone. This combination was developed into a formulation called Apatone and tested in a small clinical trial with 17 prostate cancer patients who had failed standard treatment.
Patients took 5,000 mg of vitamin C and 50 mg of K3 daily for 12 weeks. At the end of the trial, 13 of 17 patients showed slowed disease progression based on PSA markers, and there were no serious side effects. These results were preliminary, and the doses were carefully controlled in a clinical setting, which is very different from self-supplementation. The small study size means these findings are far from definitive, but they illustrate why K3 hasn’t been entirely abandoned by researchers despite its toxicity profile.