Interaction Between Warfarin And Terbinafine
A few weeks ago I was prescribed a course of oral Terbinafine to clear up a fungal infection. It seemed to work well, except that there is still some of the tinea in my toe-nails.
But over the time, I was taking the drug, I have felt that my INR was constantly wanting to slip downwards towards and below two.
Luckily I test my INR daily, and use a simple control algorithm to calculate my Warfarin dose. Normally, it is 4 mg, but if it goes below 2.3, I increase it to 5 mg, and if it goes above 2.8, I reduce it to 3 mg. So the algorithm got me taking a lot of 5 mg doses as opposed to the usual4 mg.
Only since I finished the course of Terbinafine has the INR stabilised around 2.5, which is my target value.
I didn’t at first see any link until everything settled after the course finished. But I decided today to type “Warfarin Terbinafine interaction” into Google. I found this paper from the BMJ entitled Drug points: Serious interaction between warfarin and oral terbinafine.
I think this minor incident shows the value of regular INR testing! Because I was testing daily, as the INR started to drop, my algorithm told me to increase the dose to 5 mg. In fact my average dose has gone up from 4 to 4.5 mg. in the period that I was taking the Terbinafine.
So there was no harm done at all!
The Anonymous Widower 
Sadly though, most people don’t test daily, and wouldn’t have known. I do regular blood tests, but most diabetics don’t unless they are on insulin (which I am not). I would make it a legal requirement that every diabetic did a blood test before they got behind the wheel of a car.
Comment by nosnikrapzil | April 26, 2014 |
[…] is very much a repeat of my experiences of taking Terbinafine, which I wrote about Interaction Between Warfarin And Terbinafine. Although that drug dropped the […]
Pingback by Warfarin And Lansoprazole « The Anonymous Widower | December 29, 2015 |
Antifungal drugs are notoriously bad at interacting with other drugs……..
Comment by David John Collier | March 1, 2023 |
As you can see the post was nearly nine years ago, so there was no lasting harm done.
Although, I still suffer from the occasional fungal infection, as I have since I was a teenager.
Perhaps all new drugs should be tested by people like me, who test their own INR.
Comment by AnonW | March 1, 2023 |
I developed a fungal infection, in my toe nail. On the advice of my podiatrist, I put a drop of neat tea tree oil on the toe nail morning and night. Boots do a neat version. It cleared it up – took it a while, but it worked.
Comment by nosnikrapzil | March 2, 2023