Published: 6 September 2013

Anticoagulant Effect of Warfarin Increased by Benzbromarone

Prescriber Update 34(3):28
September 2013

An interaction reported to the Centre for Adverse Reactions Monitoring (CARM) serves as a reminder to all healthcare professionals to take care when prescribing unapproved medicines.

A 78-year-old patient with ischaemic heart disease and chronic renal failure was taking warfarin for atrial fibrillation and an artificial mitral valve. The patient was started on benzbromarone for polyarticular gout. About one month after starting benzbromarone, the patient had an INR of 9.7 with widespread bruising.

Benzbromarone is a uricosuric drug that is not approved for use in New Zealand. Benzbromarone is known to selectively inhibit the metabolism of warfarin reducing its elimination and increasing its effects1. Because this medicine is unapproved, no data sheet is published on the Medsafe website.

The New Zealand Formulary (NZF) website ( ) covers unapproved as well as approved medicines. Other references also contain information on the use of unapproved medicines (eg, Martindale, Stockley’s).

Prescribers need to discuss the benefits and risks of treatment with unapproved medicines with patients (the Code of Health and Disability Services Consumers' Rights 1996). Information on the use of unapproved medicines is available on the Medsafe website (

  1. Stockley IH. 2002. Stockley's Drug Interactions. London: Pharmaceutical Press.