Warfarin + Antibiotics = Danger

Warfarin, the generic name for the most widely prescribed oral anticoagulant (blood thinner) in America, is used to prevent thrombosis (blood clots) and thromboembolism (blood clots that migrate throughout the body). People diagnosed with atrial fibrillation (rapid or irregular heartbeat), narrowed coronary arteries, who have had valve or stent replacement surgery or have a history of blood clots in the legs or lungs, often are prescribed warfarin, or its most common brand name, Coumadin.

The blood thinner can be a lifesaver for people at risk of stroke or heart attack caused by blood clots. (To learn more about maintaining heart health, see our newsletter, “A Healthy Heart: Unlocking the Key to Long Life.”)

But according to the Harvard Medical School heart health newsletter, these patients must ensure that they’re not posing additional risks by taking warfarin with certain antibiotics and antifungal drugs, which enhance warfarin’s blood-thinning effect, and possibly prompt internal bleeding.

The amount of warfarin in the bloodstream is measured by clotting time, and is expressed by the international normalized ratio, or INR. The higher the INR, the longer it takes blood to clot. In warfarin patients, medical practitioners look for a ratio of 2 to 3, although individual numbers might vary. Antibiotics affect these levels. Somebody who’s stable at 2.5 and takes an antibiotic can measure 5. If that level is sustained, it can cause gastrointestinal bleeding; a bump on the head, as the newsletter notes, can prompt bleeding in the brain.

Because antibiotics affect people differently, warfarin patients must be evaluated carefully and monitored regularly as soon as they begin a long-term antibiotic regimen. (With a short-term prescription, such as that used for two or three days in advance of dental work, an increase in INR isn’t necessarily worrisome, as the antibiotic clears the system quickly.)

Potential problems must be monitored particularly with broad-spectrum antibiotics, including erythromycin, penicillin and ciprofloxacin. Sometimes their doses must be lowered to accommodate warfarin patients.

In addition to pills, topical antibiotics-those delivered via ointment, cream or suppository-also are absorbed into the bloodstream and interfere with warfarin. Antifungal cream is a prime example. It’s often prescribed to women with vaginal yeast infections.

Like all drugs, warfarin taken alone carries potential side effects and risks. A recent story in Forbes noted that last year, warfarin was the second most prevalent drug in FDA safety reports “and has been high on the FDA list for many years.” It was the subject of 1,106 serious adverse events, including 72 deaths.

Although doctors generally discuss the risks of antibiotics with their warfarin patients, there’s plenty of opportunity for an adverse event if:

  • a patient doesn’t understand the potential significance of the drug-drug interaction;
  • a patient forgets the drug information;
  • a doctor prescribing the antibiotic is different from the doctor prescribing the warfarin, and communication between the two falters;
  • a patient doesn’t comply with INR testing;
  • the drug-interaction alert function in the doctor’s computerized medical records is inoperative and/or the medications it lists is out of date;
  • a patient uses different pharmacies for filling the warfarin and antibiotic prescriptions, so the pharmacists aren’t aware that a warning should be issued;
  • a patient gets an antibiotic sample or handwritten prescription, bypassing a computer system alert.

So both prescribing doctors and their warfarin patients both must take responsibility for managing drug use.

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