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How drugs used to reduce blood clot risk can react with certain foods

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QUESTIONI have a mechanical heart valve and take warfarin to avoid blood clots. My international normalised ratio (INR) regularly drops below the recommended level, but I’m reluctant to up my medication. I have heard that cranberry juice may have an INR-raising effect. Is this true?

ANSWER: Warfarin has, until recently, been one of the drugs most frequently associated with adverse reactions in New Zealand. Although it can prevent potentially fatal blood clots, it can also increase the risk of dangerous bleeding.

Warfarin interacts with a number of foods, prescription medicines and dietary supplements, potentially slowing or speeding up the metabolism of the drug in the body and altering its effectiveness in reducing blood clotting to a safe level.

INR testing is an integral part of warfarin treatment and measures how long it takes blood to clot. The target range for warfarin users differs, depending on the medical condition, but it’s generally between 2 to 3. INR values over 4.5 increase the risk of major haemorrhage (bleeding), and an INR less than 2 increases the risk of thromboembolism (formation of clots within the blood vessels) and associated conditions such as heart attack and stroke. For those at high risk of a blood clot, the targeted INR range may be higher, at between 2.5 and 3.5.

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Common medicines, such as antibiotics and antacids, can alter warfarin’s effectiveness, as can dietary supplements and herbal medicines such as coenzyme Q10, vitamin E, dong quai, St John’s wort, Ginkgo biloba and green tea extracts. Food and drinks such as cranberries and cranberry juice, alcohol, vitamin K-rich soybean and canola oils, spinach, broccoli, garlic and black liquorice can also have an effect.

Some published reports show cranberry juice or sauce increasing the effects of warfarin and elevating INR.

A 2011 study, published in Annals of Pharmacotherapy, details a 46-year-old woman’s adverse warfarin reaction after twice drinking cranberry juice. Her INR increased to 4.6 and 6.5 on those occasions, but it returned to a normal range once she stopped drinking the juice.

A full assessment of her case concluded that an interaction between cranberry juice and warfarin was highly probable.

Another case involved a 78-year-old man who had 1.9 litres of cranberry and apple juice over the course of a week and had an elevated INR of 6.45. Once he stopped drinking the juice, his INR returned to normal.

Although it may seem that cranberry juice was the culprit, several clinical trials have found no interaction between the drug and the berry and only one that confirmed it.

Scientists have no idea what compound in cranberries may be affecting warfarin, so there is no way to test different cranberry juices and find out why sometimes people are affected and other times not.

Warfarin is manufactured under strict controls, so each dose is identical, and with suitable monitoring by a doctor to ensure INR stays in the optimal range, it decreases the risk of blood clots.

Given there is no clear evidence that drinking cranberry juice will change your INR, it is probably safer to avoid it altogether, along with foods and supplements that may cause adverse reactions. 

This article was first published in the July 6, 2019 issue of the New Zealand Listener.