Medicines which interact with warfarin

Drug Interaction Facts. Hansten P,Horn J. Drug Interactions Analysis and Management. Drug Interaction Classification Systems. Severity - Major: The effects are potentially life-threatening or capable of causing permanent damage. Documentation: interaction is suspected, probable or established. This item is 14 years and 7 months old; some content may no longer be current.

Warfarin has the potential to interact with numerous drugs resulting in significant morbidity and potentially fatal bleeds. Loss of anticoagulant control is one of the most frequent causes of drug related hospital admissions in New Zealand and in many cases the event is precipitated by an agent which alters the anticoagulant effect of warfarin. Mechanisms include inhibition or induction of warfarin metabolism, additive or synergistic antiplatelet effects e.

Individual susceptibility may increase the risk of an interaction in some people. For many drugs the supporting evidence for an interaction is relatively weak and based on poor quality case reports. Hopefully this will be easy to read and digest and prompt the need for further advice on management when necessary. Cimetidine inhibits the metabolism of warfarin and increases its anticoagulant effect.

Ranitidine appears not to interact but there have been isolated cases of bleeding. There have been some reports of bleeding with proton pump inhibitors PPIs and warfarin but an interaction is relatively uncommon.

An awareness of the slight possibility of bleeding along with the usual monitoring of the INR are required when PPIs are given with warfarin. Anticoagulant control can be affected by changes in alcohol consumption. Poor diet and liver damage in alcoholics may also be factors. Most analgesics can increase the bleeding risk when given with warfarin but by different mechanisms. There is some evidence that the COX-2 inhibitors e. Recent case reports indicate that tramadol can inhibit the metabolism of warfarin and increase its anticoagulant effect.

Regular use of paracetamol can increase the effect of warfarin in some patients. Occasional doses of paracetamol for two to three days are unlikely to have a significant effect, but regular use for a week or more may increase the INR in some patients. Monitor the INR in people starting or stopping regular use of paracetamol.

Amiodarone inhibits the metabolism of warfarin and this interaction occurs in most patients. The onset may be slow about two weeks and it may persist for weeks or even months after the amiodarone has been stopped. The anticoagulant effect of warfarin is possibly enhanced by quinidine. Most antibiotics have been reported to alter the anticoagulant effect of warfarin and the usual mechanism proposed is reduction of vitamin K synthesis by gut flora.

Monitoring is recommended when an antibiotic is started or stopped. Clinically significant interactions appear to be more likely with ciprofloxacin, norfloxacin, macrolides e. Selective serotonin reuptake inhibitors SSRIs have antiplatelet effects and as well as increasing the risk of bleeding per se they can enhance the risk of bleeding with warfarin without increasing the INR.

The effects of warfarin are occasionally increased by venlafaxine, sometimes with an increase in the INR. With tricyclic antidepressants TCAs there are isolated reports of changes in the INR but the evidence for an interaction is poor and inconclusive. Monitor INR as normal. There are isolated reports of enhanced anticoagulant effect or hypoglycemia with sulphonylureas but evidence of a clinically significant interaction with warfarin is very weak and appears to be very unlikely. The metabolism of warfarin can be increased by carbamazepine, phenytoin and phenobarbitone reduced anticoagulant effect.

A practical guide for GPs. Research Enabling research in general practice. Increasing functionality of electronic medical records. Warfarin — indications, risks and drug interactions. A qualitative study in Australian general practice.

Barriers to diagnosing and managing hypertension. Professional Business plans. Tips from the toolkit 6. Proof of age required. Estimating age in adults without birth records. Viewpoint Health care reform. Can we maintain personal continuity? Back pages Book reviews. Back pages Clinical challenge.