p45 drugs Flashcards
What are general factors that may potentiate warfarin?
Liver disease; P450 enzyme inhibitors; Cranberry juice; Drugs that displace warfarin from albumin (e.g. NSAIDs); NSAIDs inhibit platelet function.
Do NSAIDs increase or decrease warfarin effect?
Increase - they displace warfarin from plasma albumin and inhibit platelet function.
Does cranberry juice potentiate or reduce warfarin?
Potentiate.
What are some examples of drugs that displace warfarin from plasma albumin?
NSAIDs.
Name a condition that reduces warfarin metabolism and increases INR.
Liver disease.
What is the effect of P450 enzyme inducers on INR?
They increase warfarin metabolism, decreasing INR.
What is the effect of P450 enzyme inhibitors on INR?
They decrease warfarin metabolism, increasing INR.
List some inducers of the P450 system.
Phenytoin; Carbamazepine; Phenobarbitone; Rifampicin; St John’s Wort; Chronic alcohol; Griseofulvin; Smoking.
List some inhibitors of the P450 system.
Ciprofloxacin; Clarithromycin/Erythromycin; Isoniazid; Cimetidine; Omeprazole; Amiodarone; Allopurinol; Ketoconazole; Fluconazole; SSRIs (e.g. fluoxetine, sertraline); Ritonavir; Sodium valproate; Acute alcohol; Quinupristin.
How does smoking affect warfarin metabolism?
It induces CYP1A2, increasing warfarin metabolism and decreasing INR.
Which has a greater effect on INR: chronic or acute alcohol intake?
Acute alcohol intake increases INR by inhibiting warfarin metabolism; chronic intake decreases INR.
Name two antibiotics that are P450 inhibitors and increase INR.
Ciprofloxacin; Clarithromycin (or Erythromycin).
Which P450 inducer is an antiepileptic?
Phenytoin or Carbamazepine.
Which herbal medicine induces the P450 system?
St John’s Wort.
What effect does amiodarone have on INR?
It increases INR by inhibiting the P450 system.
Is cimetidine an inducer or inhibitor of the P450 system?
Inhibitor.
What is the effect of fluconazole on INR?
It increases INR by inhibiting P450 metabolism.
Name a common SSRI that inhibits P450 metabolism.
Fluoxetine or Sertraline.
True or false: Rifampicin increases warfarin’s INR.
False – Rifampicin is a P450 inducer and lowers INR.
Which type of alcohol consumption increases INR?
Acute alcohol intake.
Practice: A patient on warfarin starts fluconazole. What happens to their INR?
It increases – fluconazole inhibits P450 metabolism.
Practice: A patient on warfarin begins rifampicin. What happens to their INR?
It decreases – rifampicin induces P450 enzymes.
Practice: Name two mechanisms by which NSAIDs potentiate warfarin.
Displace warfarin from albumin; Inhibit platelet function.
Practice: What would happen to a smoker’s warfarin dose if they quit smoking?
They might need a lower dose, as smoking induces warfarin metabolism.
