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Drug interactions between Bactrim IV and Coumadin

Results for the following 2 drugs:
Bactrim IV (sulfamethoxazole/trimethoprim)
Coumadin (warfarin)

Interactions between your selected drugs

warfarin ↔ sulfamethoxazole

Applies to:Coumadin (warfarin) and Bactrim IV (sulfamethoxazole/trimethoprim)

Using warfarin together with sulfamethoxazole may increase the risk of bleeding, especially if you are elderly or have kidney or liver impairment. You may need more frequent monitoring of your prothrombin time or INR by your doctor to safely use both medications. Call your doctor promptly if you experience any unusual bleeding or bruising, swelling, vomiting, blood in your urine or stools, headache, dizziness, or weakness during treatment with these medications. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.

Switch to professional interaction data

warfarin ↔ trimethoprim

Applies to:Coumadin (warfarin) and Bactrim IV (sulfamethoxazole/trimethoprim)

Consumer information for this interaction is not currently available.

MONITOR: Many antibacterials may potentiate the hypoprothrombinemic effect of oral warfarin and other coumarin anticoagulants. The exact mechanism is unknown but may involve inhibition of coumarin metabolism and/or depletion of certain clotting factors due to suppression of vitamin K-producing intestinal flora. Other influences such as fever, infection, malnutrition, renal impairment, age, and other concomitant underlying conditions on clotting mechanisms and anticoagulant pharmacokinetics should also be considered.

MANAGEMENT: Given the potential for clinically significant interaction and even fatality in the occasional, susceptible patient, monitoring is recommended during concomitant therapy. The INR should be checked frequently and coumarin dosage adjusted accordingly, particularly following initiation or discontinuation of the antibacterial therapy in patients who are stabilized on their anticoagulant regimen. Patients should be advised to promptly report any signs of bleeding to their physician, including pain, swelling, headache, dizziness, weakness, prolonged bleeding from cuts, increased menstrual flow, vaginal bleeding, nosebleeds, bleeding of gums from brushing, unusual bleeding or bruising, red or brown urine, or red or black stools.


  1. Ministerio de Sanidad y Consumo. Gobierno de España "AEMPS. Agencia Española de Medicamentos y Productos Sanitarios. Available from: URL: https://sinaem4.agemed.es/consaem/fichasTecnicas.do?metodo=detalleForm."
  2. Cerner Multum, Inc. "UK Summary of Product Characteristics." O 0

Drug Interaction Classification

The classifications below are a guideline only. The relevance of a particular drug interaction to a specific patient is difficult to determine using this tool alone given the large number of variables that may apply.

Major Highly clinically significant. Avoid combinations; the risk of the interaction outweighs the benefit.
Moderate Moderately clinically significant. Usually avoid combinations; use it only under special circumstances.
Minor Minimally clinically significant. Minimize risk; assess risk and consider an alternative drug, take steps to circumvent the interaction risk and/or institute a monitoring plan.

Do not stop taking any medications without consulting your healthcare provider.

Disclaimer: Every effort has been made to ensure that the information provided by Multum is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. Multum's information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill, knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate that the drug or combination is safe, effective, or appropriate for any given patient. Multum Information Services, Inc. does not assume any responsibility for any aspect of healthcare administered with the aid of information Multum provides. Copyright 2000-2015 Multum Information Services, Inc. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the drugs you are taking, check with your doctor, nurse, or pharmacist.