ACETAMINOPHEN INTERACTIONS WITH WARFARIN
Brand Names: ACETAMINOPHEN - Tylenol, Feverall, Acephen, Neopap, Apacet, Children's Dynafed Jr. Genapap, Maranox, Mapap, Panadol, Tempra, Aceta, Aspirin Free Pain Relief, Genebs, Meda, Tapanol, Aspirin Free Anacin Maximum Strength, Extra Strength Dynafed E.X. Redutemp Dapacin, Infants' Pain Reliever, Silapap, Ridenol, Oraphen PD, Children's Pain Reliever, Halenol, Liquiprin, Capital with Codeine, Tylenol with Codeine, Acetaminophen with Codeine, Aceta with Codeine, Fioricet with Codeine, Lortab, Zydone, Hydrocodone with Acetaminophen, Bancap HC, Ceta-Plus, Co-Gesic, Duocet, Dolocet, Hydrocet, Hydrogesic, Hydrophen, Margesic - H, Lorcet, Anexsia, Panacet, Stagesic, T-Gesic, Vicodin, Norco, DHC Plus, Acetaminophen with Oxycodone, Percocet, Roxicet, Roxilox, Tylox, Propoxyphene Napsylate with Acetaminophen, Darvocet N, Propacet 100, Wygesic, and many other brands. There are so many that they could not all be listed here. If you are taking warfarin, you should read the label of any pain relief medication to determine if it contains acetaminophen. If you are unsure, purchase the product at a pharmacy where you can have the pharmacist check the label for you.
Acetaminophen is the safest pain reliever to take with warfarin. It does not cause bleeding of the lining of the gastrointestinal tract as many other pain relievers do.
However, Hylek et al. found that if a person takes more than 26 regular- strength (325 mg each) or 19 extra-strength (500 mg each) over one week, the International Normalized Ratio (INR) may be raised. This will increase the chance of a person bleeding.
Two articles have added insight as to how this may occur. Lehman hypothesized that various enzymes in the liver may be involved in different patients. These have to do with metabolizing the two forms of warfarin and acetaminophen. These may come into play with varying conditions such as in older patients, those with atrial fibrillation and congestive heart failure. This theory could account for why some people are affected and some are not.
Whyte et al. reported their observations in a regional toxicology center. They found that a small rise in the INR is common after acetaminophen poisoning without liver injury. This appears to be caused by inhibition of clotting factor VII. This would account for the interaction between acetaminophen and warfarin.
My opinion is that the theories of Lehman and Whyte do not contradict each other. They are probably both involved in explaining what Hylek observed.
Addendum: Many of you may be familiar with other websites and a story that ran on CNN stating that acetaminophen and warfarin is a harmful combination. Since I wrote this page, I have had an opportunity to speak with Dr. Hylek. She regretted giving the interview, which was edited to make it appear that she was stating that this combination was harmful.
References:
Hylek EM et al. Acetaminophen and other risk factors for excessive warfarin anticoagulation. JAMA 1998;279:657-662.
Lehmann DE. Enzymatic shunting: resolving the acetaminophen-warfarin controversy. Pharmacotherapy 2000; 20:1464-1468.
Whyte IM et al. Acetaminophen causes an increased International Normalized Ratio by reducing functional factor VII. Ther Drug Monit 2000;742-748.
Remember that nothing on these pages should encourage you to start or stop taking any medication. You should consult with your physician. Many interacting medications can be safely taken if the INR is monitored frequently. Stopping an interacting medication can upset the balance between warfarin and other medications.