Ajazzy
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There is an interesting interaction between Phenytoin and Oral anticoagulants, What is It guys? lets brush up
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| docarjun
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whats it?
phenytoin displace wasrfarin form plasma protein binding....so enhance effect.
whats the answer...
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| Idiopathic
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Phenytoin speeds up metabolism of warfarin. Leads to decreased INR and hyper-coagulability?
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| mash
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phenytoin induces p450 so, it enhances metabolism of warfarin(which is the substrate fr same enz)
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| docarjun
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i got it..
hey mash..phenytoin does both....competition for plasma protein binding and induction of p450...so it will decrease the effect of warfarin
right?
what i read in my textbook(in india)
in antiepileptic section
warfarin inhibit phenytoin metabolism..can lead to toxicity
and
in blood section
phenytoin inhibit warfarin metabolism.
actually i was waiting for some discussion...can u please clear this to me?
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| mash
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actually wat i know is dat 1) both phenytoin n warfarin r metabolized by cyt P450(its either 3A or 2B, i dont remember exactly)
2)phenytoin induces this enzyme
3)warfarin is weakly bound to plasma but binding is extensive
so, wat i can make out is
if pt is already on phenytoin and he is given warfarin then it wud increase warfarin' metabolism (bcoz of already induced enz) and decrease its effectiveness
and if pt is on warfarin and then phenytoin is started ,it wud displace warfarin frm plasma proteins and thus wud increase its effect..but this shud be transient bcoz when phenytoin will induce cyt P450, warfarin metabolism wud be enhanced and thus decreased effect.
(though i m not sure)
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I hear and I forget. I see and I remember. I do and I understand.
--Confucius
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| docarjun
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:icon_scratch:
umm.......i am sill not satisfied.
I dont have my textboks with me now.Rather i doont have any textbook.
If u r referring katzung or any standard textbook....please just give me what the book is saying .So i will try to memorise it.
Ofcourse u might have read this before in ur textbook only.
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| Ajazzy
Forum Newbie
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Lets keep it short folks, the fact I remembered the question was just to brushh up on two important reason of drug interaction and in this example you find both.
A patient on Warfarin if given Phenytoin initially will show an increase in bleeding tendencies due to displacement of Warfarin from protein binding (ususally warfarin is 98% protein bound) and increase warfarins effect. Later as Phennytoin induces CYT3A4 (CYT P450) it leads to a reduction i warfarin effect so.... there it is! I hope that brushes up a few things
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| docarjun
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yes....got it!
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