Warfarin interaction with medications

Tam LS, Chan TYK, Leung WK, Critchley JAJH. Warfarin interactions with Chinese traditional medicines danshen and methyl salicylate medicated oil. 

Tam, L.S., T.Y.K. Chan, W.K. Leung, and J. Critchley. 1995. Warfarin interactions with Chinese traditional medicines Danshen and methyl salicylate medicated oil. Intern. Med.. 25(3) 258. 

Inform the patient about the potential drug-drug interactions with warfarin, including over-the-counter medications and dietary supplements (Tables 7-8, 7-9, and 7-10). Instruct the patient to call the health care practitioner responsible for monitoring warfarin therapy before starting any new medications or dietary supplements. 

Several studies have evaluated dietary supplements such as isoflavones, which are found in soy products and red clover. A well-controlled trial in more than 400 postmenopausal women evaluating a specific isoflavone, ipriflavone, found no benefits on bone mineral density or fracture rates after 3 years.47 Nevertheless, because these therapies are available without prescription and are not regulated by the FDA, patients may choose to self-medicate with isoflavones. Lymphocytopenia appeared in several patients treated with ipriflavone in clinical trials. Additionally, ipriflavone should be used with caution in immunocompromised patients or those with renal disease. It may inhibit CYP1A2 and CYP2C9 and may interact with drugs metabolized by those pathways, such as warfarin. 

Oral azoles are associated with significant interactions, particularly due to cytochrome P-450 isoenzymes. Medications that interact with azoles include warfarin, phenytoin, theophylline, rifampin, cyclosporine, and zidovudine. For patients receiving only a few doses, these interactions do not pose a significant risk. These interactions may pose a risk for patients receiving long-term suppressive therapy for recurrent infections. 

Pharmacokinetic interactions Preliminary evidence suggests that Saint-John s-wort induces the cytochrome oxidase enzyme isoform CYP3A4 (Ernst 1999). This raises the potential for pharmacokinetic interactions with drugs metabolized by the same enzyme. A few cases have been reported of reduced warfarin levels (Yue et al. 2000). Similar interactions have also been reported for concurrent use with digoxin, theophylline, and cyclosporin (Nebel et al. 1999 Ruschitzka et al. 2000 Johne et al. 1999). As with any other medication, potential interactions should be considered when taking a combination of drugs. 

Clinicians from Hong Kong reported a case of potential danshen-warfarin interaction in a 48-year-old female with a history of rheumatic heart disease, atrial fibrillation, and mitral stenosis (11). The patient underwent successful transvenous mitral valvuloplasty for management of her medical conditions, and was discharged with 1 mg warfarin, as well as... 

Simultaneous administration of sucralfates with other nonantacid medication should be given after an interval of at least 2 hours. Sucralfates interact with rantidine, cimetidine, digoxin, quinidine, theophylline, and warfarin. Sulfasalazine should be administered with caution with antibacterials and antineoplastics.179... 

In two of the five spontaneous bleeding episodes described in Heading 4, medications that can affect platelet function or prothrombin time (PT) (i.e., aspirin and warfarin) were involved. Because GB is known to be an inhibitor of PAF (41), in theory GB could interact with antiplatelet drugs (e.g., aspirin, nonsteroidal anti-inflammatory drugs, clopidogrel, ticlopidine, dipyridamole) or anticoagulants (e.g., warfarin, heparin). EGb 761 was shown to potentiate the antiplatelet effect of ticlopidine in rats (42). However, in two studies in humans, the coadministration of GB with warfarin had no effect on either international normalized ratio or warfarin metabolism (43,44). 

This medication interacts with aspirin, warfarin, erythromycin, and theophylline therefore, this statement warrants further intervention by the nurse. 

Since erythromycin complexes and inactivates drug oxidizing systems such as cytochrome P-450, it has the potential to alter the metabolism of other drugs. The metabolism and excretion of theophylline, warfarin, carbamaz-epine, and methylprednisolone are inhibited by erythromycin [283-286]. As a potent antibiotic, it can also affect metabolism by gut micro-organisms of drugs such as digoxin. At least some of the newer derivatives may cause fewer drug interactions and thus may be better tolerated if co-administered with medications for other illnesses [287-289]. 

Meloxicam has been shown to have interactions with the following common medications ACE inhibitors, aspirin, cholestyramine, cimetidine, digoxin, furo-semide, hthium, methotrexate, warfarin [ 1 ]. 

Warfarin was also implicated in several interactions with antiviral medications. A recent case series suggests an interaction between warfarin and oseltamivir (Tamiflu). Among 15 patients on anticoagulation tiierapy who took oseltamivir, 7 had an increase in INR levels and the remaining 8 maintained stable INR levels. Bleeding events such as blood-tinged sputum and bloody ascites occurred in three patients in the increased INR group [13 ]. 

Use of zileuton is uncommon due to the need for dosing four times a day, potential drug interactions, and the potential for hepatotoxicity with the resulting need for frequent monitoring of liver enzymes. In patients started on zileuton, serum alanine aminotransferase concentrations should be monitored before treatment begins, monthly for the first 3 months, every 2 to 3 months for the remainder of the first year, and then periodically thereafter for as long as the patient continues to receive the medication. Zileuton also inhibits the cytochrome P-450 (CYP) mixed function enzyme system and has been shown to decrease the clearance of theophylline, R-warfarin and propranolol.34... 

A contact dermatitis occurs infrequently. Because feverfew also inhibits human blood platelet aggregation, interactions are possible with antithrombotic medications such as aspirin or warfarin (Groenewegen and Heptinstall 1990). Abrupt discontinuation of feverfew by people taking it chronically for treatment of migraine can produce rebound withdrawal symptoms. These consist of migraines, anxiety, poor sleep patterns, and stiffness of the muscles and joints. 

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