Vitamins: They’re good for you, right? In general, the answer is a resounding yes. But, that’s not the case with vitamin K3.

Vitamin K3 is a synthetic version of vitamin K and is actually toxic to humans. For that reason you’ll never see it as a supplement on store shelves.

Here’s everything you need to know about vitamin K3 — plus the details on the kinds of K you actually need.

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K3 sounds like a mountain peak in the Himalayas that rivals K2, but vitamin K3 (aka menadione) is actually a synthetic version of vitamin K.

Two forms of vitamin K — K1 and K2 — can be found naturally in food, unlike vitamin K3, which is created in a lab.

Vitamin K has several important jobs in your body. It’s most famous for helping your blood clot, which keeps you from bleeding excessively from wounds large and small. (Fun fact: The “K” comes from the German word “koagulation.”)

Both K1 and K2 are involved in blood clotting, but K2 goes the extra mile.

“Vitamin K2 also improves bone quality, is involved in calcium transport, and prevents calcium deposition in blood vessel walls,” says dietitian Kris Sollid, RDN, senior director of nutrition communications at the International Food Information Council.

You can load up on both vitamin K1 and vitamin K2 through a variety of foods. K1 comes from plants — primarily leafy greens and a few oils. K2, on the other hand, is found in animal products and some fermented foods. These forms of vitamin K are also found in multivitamins.

Mmm-kay, so what’s so wrong with the synthetic form of vitamin K? Plenty of other vitamins get created safely in labs. Why is K3 different?

Unfortunately, artificial vitamin K3 isn’t just unnatural, it’s actually harmful to humans. Although it was sometimes used as a dietary supplement in the 20th century, research on K3 in the ’80s and ’90s revealed that it could cause liver damage.

In fact, as an injection, K3 has been known to induce liver toxicity, jaundice, and anemia from ruptured blood cells. (Yeah, we’ll pass on all that, thanks.)

Another unpleasant side effect? K3 can interfere with one of your body’s naturally occurring antioxidants, glutathione. As a result, it can increase oxidative damage to your cells — the exact opposite of all that antioxidant-rich eating you may try to do.

Considering all these concerns, it’s no surprise that K3 isn’t sold as a dietary supplement. But since K1 and K2 in food and supplements have your bases covered, you definitely don’t need to worry about missing this artificial vitamin.

P.S.

Though you won’t see it as an ingredient in your gummy vitamins, menadione is sometimes used in animal feed. But don’t worry about Fido. In FDA-approved doses, K3 appears to be nontoxic for animals.

Putting aside its harmful effects for a sec, vitamin K3 does have some possible silver linings.

A 2020 study found that K3 has “potent anticancer activity” for some cancers of the breast, kidneys, liver, prostate, skin, and cervix. Plus, research in 2019 suggested that K3 could suppress some activities of colorectal cancer cells.

Vitamin K3 might also be a useful antibacterial agent — particularly against H. pylori, a nasty bug that can grow in your stomach and cause ulcers.

In a 2019 study, K3 inhibited the growth of H. pylori (although this research was performed in test tubes, not in humans).

The data on these effects is still pretty limited, and more research is needed before we can make any conclusions. And K3 is definitively known to pose a risk of liver damage.

Bottom line: Still don’t take K3

Future research may find ways of avoiding K3’s harmful effects so it can be used as an antibacterial or anticancer treatment. But in the meantime, it’s important NEVER to self-treat with menadione in any form, since we know it’s harmful.

The amount of vitamin K you need varies by age and gender, but for both men and women, a little goes a long way. In fact, the daily Adequate Intake for men age 19 and older is just 120 micrograms (mcg). Women in the same age range need at least 90 mcg per day.

Compared with your daily requirements for many other vitamins and minerals — which come in much larger measurements of grams or milligrams — your vitamin K needs are pretty small. Still, it’s important to get enough of this blood-clotting, bone-building nutrient through healthy foods.

Vitamin K injections are sometimes used in medical applications. All newborns get a vitamin K injection, which the CDC recommends because infants are born with very low levels.

Vitamin K levels are generally not monitored, except in patients who have bleeding disorders or are on anticoagulants. Vitamin K supplementation may be used to manage these conditions.

Here’s a general rule for vitamin K-rich foods: K1 comes mostly from plants, K2 mostly from animals. Try any of the following foods to get your fill of K for the day.

Foods high in vitamin K1, per 1/2-cup serving:

  • Kale, cooked: 531 mcg (443% DV)
  • Mustard greens, cooked: 415 mcg (346% DV)
  • Broccoli, cooked: 110 mcg (92% DV)
  • Brussels sprouts, cooked: 109 mcg (91% DV)
  • Spinach, raw: 73 mcg (60% DV)

Foods high in vitamin K2, per 3.5-ounce serving:

  • Natto: 1,062 mcg (885% DV)
  • Pork sausage: 383 mcg (320% DV)
  • Beef liver: 106 mcg (88% DV)
  • Hard cheese: 76 mcg (63% DV)
  • Pork chops: 69 mcg (57% DV)

How your body absorbs vitamin K from food

Once you’ve downed a dinner of pork chops and greens or natto and broccoli, your body gets to work absorbing and distributing vitamin K.

“Vitamin K is absorbed in the small intestine, concentrated in the liver, and dispersed throughout the body to various tissue and organs (e.g., liver, brain, heart, pancreas and bone),” Sollid explains. “Vitamin K is also produced by the bacteria in our gut.”

Looking for ways to boost absorption? Because vitamin K is fat-soluble, adding a source of fat will enhance your uptake — so go ahead and sauté kale in olive oil or dip broccoli in some creamy ranch dressing.

Certain dietary factors can also affect the amount of vitamin K your body soaks up. “A couple of examples that interfere with vitamin K absorption are low fat diets and extended use of some medications (e.g., antibiotics),” Sollid notes.

Besides that, not all vitamin-K rich foods deliver the goods equally. According to Sollid, vitamin K’s bioavailability is greater (meaning your body can more easily absorb and use it) when it comes from plant oils than when it comes from other plant foods.

For good health, it’s critical to keep up with your K in the form of foods or a vitamin K1 or K2 supplement. As for K3, considering its toxicity to humans, it’s for the birds (or the dogs or the cats or… you get the idea).