All about vitamin K when taking warfarin (coumadin)

Those taking a blood-thinning drug such as Coumadin (otherwise known as Warfarin) are most likely wondering how taking this drug affects other things in your life- such as eating foods containing vitamin K.  A great description of the basics of taking an anticoagulant drug, are listed below per the company ‘Life Extensions’:

Patients taking warfarin must be regularly screened by their physician to make sure that their INR (international normalized ratio, a measure of how quickly blood coagulates, or clots) remains within a safe range.  This delicate balance is difficult to obtain, because one’s INR can change due to alterations in one’s diet or other medications. Too many green vegetables at one meal can upset one’s INR. Great care must be taken by those on warfarin in order to achieve INR stability. Usually those on warfarin must dutifully report to their physician’s office every two to three weeks for a standard blood test to make sure that they are in the safe range. Too high or too low an INR reading can quickly lead to complications. For example, when the warfarin dose is too low (and the INR is below the reference range), blood is apt to clot within blood vessels, choking off blood supply to vital organs such as the lungs, heart muscle, or brain. But when the dose is too high (i.e., the INR is above the reference range), excessively reduced clotting capability of the blood may result in bruising and dangerous bleeding—into the gastrointestinal tract, urine, and even the brain—that can be lethal.  To obtain the greatest benefits of treatment, the patient’s INR must be ideally maintained within a therapeutic range, usually 2-3 (but the range may vary). Certain medications, including cholestyramine, colestipol, mineral oil, and orlistat may decrease the absorption of vitamin K.   Large doses of salicylates such as aspirin may result in vitamin K deficiency.  Cephalosporin antibiotics can cause vitamin K deficiency.’  (3)

Although there are different forms of vitamin K, we will focus on the one that most people on blood thinners have heard about- vitamin K1 (phylloquinone), which is the form that is found mainly in dark green leafy vegetables.  Vitamin K1 is an essential and needed nutrient for the body that also plays a large role in blood clotting.  Vitamin K1 is also an antagonist to the blood-thinning effects of Warfarin since vitamin K1 helps to clot the blood.  As said in the above paragraph, the body is always trying to maintain a delicate balance of the bloods consistency.

Some main food sources of vitamin K1 are listed in the chart below found on the ‘National Health Institute’s Clinical Data’(1):

Food                                        Serving Size                  Vitamin K (mcg)

Kale (cooked)                                ½ cup cooked                       531

Spinach (cooked)                            ½ cup                                  444

Collards greens (raw)                     1/2 cup                                 418

Swiss chard (cooked)                     ½ cup                                   299

Mustard greens (raw)                     1 cup                                    279

Turnip greens (cooked)                  ½ cup                                   265

Parsley, (raw)                                  1/4 cup                                246

Broccoli (cooked)                             1 cup                                   220

Brussels sprouts (cooked)                1 cup                                   219

Mustard greens (cooked)                 1/2 cup                                210

Collards (raw)                                   1 cup                                   184

Spinach (raw)                                    1 cup                                   145

Turnip greens (raw)                          1 cup                                    138

Endive (raw)                                     1 cup                                     116

Broccoli (raw)                                    1 cup                                     89

Cabbage (cooked)                            1/2 cup                                   82

Green leaf lettuce                              1 cup                                      71

Prunes (stewed)                                1 cup                                      65

Romaine lettuce (raw)                       1 cup                                      57

Asparagus                                         (4 spears)                               48

Avocado                                             1 cup (cube, slice, puree)      30-48

Tuna (canned in oil)                           3 ounces                                37

Blue/black-berries (raw)                     1 cup                                      29

Peas (cooked)                                   ½ Cup                                     21

*Though everyone’s needs are different, it is said that about 85 mcg is the limit of vitamin K you should be consuming each day while on a blood thinning medication, unless your doctor recommends otherwise.  This amount is based off of the recommended daily allowance for vitamin K/day.

The chart below is found off of the ‘care clinical research’ website showing the relationship between Warfarin and other drugs/vitamins.  ‘Warfarin is monitored by the one stage prothrombin (blood clotting) time.  Prothrombin times are reported in seconds, as a ratio of the prothrombin time in seconds to the mean normal prothrombin time of the laboratory, and as the international normalized ratio (INR).  The INR is the most reliable way to monitor the prothrombin time.’(4)

Some Drug Interactions With Warfarin

Drugs That May Lengthen PT Drugs That May Shorten PT
(Higher INR; increased warfarin effect) (Lower INR; decreased warfarin effect)
Antibiotics Anti-inflammatories Alcohol Penicillin
Carbenicillin Allopurinol Antacids Rifampin
Erythromycin Fenoprofen Antihistamines Spironolactone
Fluconazole Ibuprofen Barbiturates Sucralfate
Isoniazid Indomethacin Carbamazepine Trazodone
Ketoconazole Naproxen Cholestyramine Vitamin C
(large doses)
Metronidazole Phenylbutazone Griseofulvin
Moxalactam Piroxicam Haloperidol
and other
Sulfinpyrazone Oral contraceptives
Trimethoprimsulfa Zileuton
Anabolic steroids Omeprazole
Cimetidine Phenytoin
Clofibrate Tamoxifen
Disulfiram Thyroxine
Lovastatin Vitamin E (large doses)

Other factors affecting prothrombin time:

Conditions that interfere with vitamin K uptake or interfere with liver function will increase the warfarin effect:

  • Expect a longer blood clotting time with congestive heart failure, jaundice, hepatitis, liver failure, diarrhea, or extensive cancer or connective tissue disease.
  • Expect a longer blood clotting time when receiving warfarin and are hospitalized for any reason.

Metabolic alterations can affect the prothrombin time

  • Expect a longer blood clotting time with hyperthyroidism or high fever.
  • Expect a shorter blood clotting time with hypothyroidism. (4)

*How hydrated you are also affects the bloods ability to clot (dehydration leads to thicker blood- to stay healthy, stay hydrated!) (2)

*Drinking alcohol is also said to affect INR, the best option is to not drink at all, but if you must drink- don’t do any ‘binge’ drinking, rather, drink a small amount.  If drinking daily- plan approximately the same minimal amount every day and make aware whomever it is that is working with you on your daily Warfarin dose.  (Alcohol is known to interact with medications) (5)

Dietary supplements and herbal medications:

Many dietary supplements can alter the INR/PT such as: arnica, bilberry, butchers broom, cat’s claw, dong quai, feverfew, forskolin, garlic, ginger, ginkgo, horse chestnut, insositol hexaphosphate, licorice, melilot (sweet clover), pau d’arco, red clover, St. John’s wort, sweet woodruff, turmeric, willow bark, and wheat grass.  Much is unknown about the effects of the combination of certain drugs with certain supplements.  It helps when you are able to work with a physician that is familiar on how to correctly combine supplements and drugs.  The safest policy is for individuals on warfarin (Coumadin) to avoid all dietary supplements until your physician approves.  This includes any vitamin/mineral supplements that list vitamin K on the label.  If they are taken regularly on a daily basis, they pose less of a problem than if taken off and on. (1)

In conclusion- traditionally, patients taking warfarin have been advised to avoid vitamin K to prevent excess clotting.  Scientists have now discovered that regular consumption of a modest amount of vitamin K actually helps stabilize the anticoagulant effects of warfarin.  For more help regarding the diet aspect, speak with the Registered Dietitian here at the Heart and Vascular Institute in Lapeer.


1.)  Important information to know when you are taking: Coumadin (warfarin) and vitamin K. (n.d.). Retrieved April 27, 2015, from

2.)  Blood Clots: Deep Vein Thrombosis. (n.d.). Retrieved April 27, 2015, from

3.) Barclay, L. (n.d.). Vitamin K & Warfarin. Retrieved April 27, 2015, from

4.) (n.d.). Retrieved from

5.) Alcohol misuse, genetics, and major bleeding among warfarin therapy patients in a community setting.

(2015, April 8). Retrieved April 28, 2015, from


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