Three Challenges in the Art of Prescribing Warfarin

The blood thinner we have used for so many years is gradually being replaced by the novel anticoagulants, which don’t require laboratory monitoring and have fewer interactions. But for some indications, warfarin is still preferred and for many patients, it is still by far the more affordable anticoagulant.

Dosing warfarin has always been an art and it seems to be less often mastered than it used to be. The three challenges are drug interactions, food interactions and dosing schedules.


Just the other day, I was covering for a colleague and got an urgent message that her patient had a supratherapeutic INR – too much thinning of his blood. I asked the medical assistant to find out if the patient was taking any new medications, like ciprofloxacin, that might interact with the warfarin. I just threw that drug name out because it is such a common and overlooked interaction. Sure enough, somebody else had prescribed ciprofloxacin two days earlier for a urinary infection.

I played detective and tracked down the urine culture, which showed the coli bacteria were resistant to ciprofloxacin, but sensitive to nitrofurantoin (safe) and Bactrim (unsafe). I messaged the prescribing provider, who changed the patient’s antibiotic to nitrofurantoin, so I just ordered the warfarin held for two days.

Many providers seem to be unaware or less paranoid than I am about drugs that interact with warfarin. I once had a patient end up in the intensive care unit with critical internal bleeding because I prescribed levofloxacin with plans to check her INR every couple of days during her antibiotic course. That was clearly not cautious enough in her case.

I have seen great variability in how much other drugs affect the effect of warfarin, especially azithromycin, amoxicillin-clavulanate and also acetaminophen and prednisone, both of which in most people doesn’t seem to cause much trouble. But I worry about all of them, plus sulfamethoxazole, metronidazole, fluconazole, NSAIDs (obviously) and new starts of amiodarone, sertraline, carbamazepine and many others. Over the counter agents to worry about include fish oil, ginkgo biloba and St Johns Wort.

This is not a complete listing, and since most of us have EMRs that warn us of interactions you would think close calls like this would never happen. The problem here is the multitude of basic warnings providers know in their sleep, so that the less famous issues drown among the unnecessary alerts (see my posts about Alarm Fatigue).

I end up using epocrates’ interaction checker on my iPhone to double check sometimes, but, as I said, I’ve been burned so I know this stuff know.


Warfarin interferes with the role of vitamin K in the coagulation process. Therefore, if you flood your system with foods rich in vitamin K, which is the pharmacological antidote to warfarin, you decrease the effectiveness of warfarin. A week before my ciprofloxacin case, one of my own patients suddenly had a low INR. “Ask him if he’s been eating fiddleheads”, I told Autumn. Sure enough, this Maine spring delicacy was the culprit. The season is short and he wasn’t going to have more, he said, so I didn’t change his dose schedule.

A lot of people are under the impression they cannot eat green vegetables while on warfarin. I tell them that’s like saying you can’t open your windows in the winter if you heat with wood. Imagine you know how many logs to put in the wood stove at certain outdoor temperatures. Then imagine you decide to open a window now and then. You would then have to adjust your fire whenever you opened the window, compensating for the heat loss. If you instead decided to leave a crack open all the time, you would quickly figure out your new firewood budget.

So I simply tell my patients, “eat all the greens you want, but be sure to keep the amount the same every day”.


Mr. Magoo is like me without my glasses. If I were to drive in a snowstorm without my glasses, I would only see a couple of feet in front of me and I would be turning my steering wheel a lot more than necessary. I wouldn’t be able to tell if I was entering a small or a big curve, for instance. If I could see further ahead, I would make smaller corrections. Many providers will look at the current INR value and the previous one, and the current warfarin dose. Then they change the dose. Unless you have a flowsheet that tells you that the last time you made that change, bad things happened, you will make the same poor choice again. On paper, such flowsheets are easy to maintain, but – believe it or not – in many EMRs it is just too darn cumbersome to do.

I have a three ring binder with all my warfarin patients’ flowsheets. It helps me avoid Mr. Magoo type errors and it also serves as a low tech way of making sure no warfarin patients fall off my radar screen. Autumn or I sometimes just flip through the binder to make sure our flock doesn’t wander off, so to speak.

An old fashioned method of managing an old fashioned medication…

2 Responses to “Three Challenges in the Art of Prescribing Warfarin”

  1. 1 johndykersmddykerscom June 7, 2021 at 2:57 pm

    A very compliant older gentleman kept bouncing around with his INR. Finally learned he was moving between the 3 daughters whose household diets were very different!

  2. 2 Nell Nestor June 8, 2021 at 8:47 pm

    When I was in practice it was a never ending amazement to me how cavalier some folks were about prescribing to folks on warfarin. It ain’t a benign drug, folks.

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