possible drug interaction ?

[Ama2007]

hi,


I have a question regarding warfarin.

I have checked the BNF and it says paracetamol interacts with warfarin (rare?)...but does it make a difference if its regular or as required paracetamol?

and if there is a clinical interaction would you decrease warfarin dose?


I have read about it on the net but i want a proper book about warfarin interaction and what action you can take in certain situations...any recommendations (this is just for my finals).

[Nik]

In the context of analgesia it's a better choice than NSAID's because it doesn't affect platelet activity or cause bleeding. It's a funny old potential interaction - from what I remember it doesn't cause a significant rise in INR and it's probably one to just keep an eye on. Best to check the most up to date Stockley's available for more detailed info.

[Ama2007]

I found regular pro-long use can potentiate the effect of warfarin but intermittent use doesnt from the SPC website and the BNF. would you do anything with dose if the INR IS 3.6 (2-3) for DVT...AND patient taken regular paracetamol for knee pain. what would you do ?

[Nik]

This all depends on whether pt needs regular paracetamol to control pain. If not, then can advise prn paracetamol or adjust warfarin dose accordingly. As mentioned check Stockley's, the bible of drug interactions. Also need to consider other factors which can affect INR, not just drugs.

[crit care]

you may need to rule out other causes for raised INR, was it just because they took too much warfarin, or missed a dose and so soubled up. what other stuff had they taken (think of OTC products they may ahve self medicated with)

i don't get hung up on paracetamol and warfarin, also depends which was started first. if you are adding in warfarin to existing therapy then whetehr they interact or not, you can titrate the warfarin from day 1 and be in control, its only if you do it the other way round ie add therapy to warfarin, then you can adjust the warfarin accordingly to the interaction

i wouldn't have thought paracetamol would significantly change the effects of warfarin, but i'm coming from a hospital environment where INR is checked every few days if not daily

[Ama2007]

another question...

A 68 years old lady presents with an INR of 6.2. She reports feeling slightly unwell since she had the flu jab 2 days previously.

do you just withold dose until return back to target range? and when do you refer to a warfarin patient to a doctor for review?

.....last one...I m not too sure about this one.

A patient with AF is found to have INR 3.6(2-3) On questioning she reports that carbamazepine, which had been using for neuropathic pain following shingles, has now been stopped.

do you withold dose becasue its outside range or do you increase dose because carbamazepine is likely to reduce warfarin effect, will it still have an effect after its been stopped?


thanks

[crit care]

For the first case, you may want to reverse the INR as she is at increased risk of bleeding. Is the patient actively bleeding or not? Possibly vitamin K to reverse the INR..what is she taking warfarin for? If the INR drops too low post reversal may want to think about anticoagulant cover until INR gets back into range..I wouldn't want someone with a metallic heart valve to not have proper anticoagulation on board!

for case 2, since she is no longer on carbamazepine and INR is raised, poss withold a dose or reduce the dosage and retest INR

For both cases, you need to establish why INR is raised, from case 1 i can only assume it is because of the flu jab based on the info provided..but think why else it could be raised..has the patient simply taken too much warfarin, same in case 2....if carbamazepine reduces effects of warfarin, was the dosage increased whilst they were taking this to take the interaction into account and then on stopping the carbamazepine was the warfarin dosage not adjusted?

[Nik]

In case 1 would you not consider vit K only if she was actually bleeding at that INR ? My first thought would have been to withhold the dose of warfarin, increase freq of monitoring and restart when INR drops below 5, and if pt had additional risk factors for bleeding then give 1mg vit k in addition. Anyway those were my thoughts.

[crit care]

you can use vitamin K in bleeding and non bleeding patients...it would be quite unsafe to maintain that INR and you don't know how long it will take to fall, if it will!

[johnep]

Important that the Vit K is K1(phytomenadione) and not a synthetic analogue.
johnep