Is nitrofurantoin safe to use with warfarin (Coumadin)?

From the Guidelines

Nitrofurantoin can be used with warfarin, but close monitoring of INR is recommended due to the potential interaction. The interaction between nitrofurantoin and warfarin is not explicitly mentioned in the provided study 1, but it is known that all antibiotics can alter the gut microbiome, which is a rich source of vitamin K, and thereby potentiate anticoagulant effects of warfarin. Patients taking both medications should have their INR monitored more closely, especially when nitrofurantoin is started or stopped.

Key Considerations

• Nitrofurantoin is an antibiotic commonly used to treat urinary tract infections, while warfarin is an anticoagulant that prevents blood clots.
• The interaction between these medications is not considered major, but nitrofurantoin may occasionally affect vitamin K-producing bacteria in the gut, which could theoretically alter warfarin's effectiveness.
• Patients should inform their healthcare provider if they notice any unusual bleeding or bruising while taking both medications.
• It's also important to complete the full course of nitrofurantoin as prescribed, typically 3-7 days for uncomplicated UTIs, even if symptoms improve before completion.
• Staying well-hydrated while taking nitrofurantoin can help reduce the risk of kidney-related side effects, as noted in general clinical practice, although not specifically mentioned in 1.

Monitoring and Precautions

• Close monitoring of INR is crucial when using nitrofurantoin with warfarin, as the interaction can increase the risk of bleeding.
• Healthcare providers should be aware of the potential interaction and adjust the warfarin dose accordingly, as pre-emptive warfarin dose reductions are recommended for certain antibiotics, such as sulfamethoxazole and metronidazole, as mentioned in 1.
• Patients should be educated on the signs and symptoms of bleeding and the importance of reporting any unusual bleeding or bruising to their healthcare provider.

From the FDA Drug Label

Pharmacodynamic mechanisms for drug interactions with warfarin sodium tablets are synergism (impaired hemostasis, reduced clotting factor synthesis), competitive antagonism (vitamin K), and altered physiologic control loop for vitamin K metabolism (hereditary resistance) Potential drug interactions with warfarin sodium tablets are listed below by drug class and by specific drugs. Medications of unknown interaction with coumarins are best regarded with caution.

The FDA drug label does not answer the question.

From the Research

Interaction between Nitrofurantoin and Warfarin

• There is no direct evidence in the provided studies that specifically addresses the safety of using nitrofurantoin with warfarin 2, 3, 4, 5, 6.
• The studies primarily focus on the efficacy and safety of nitrofurantoin in treating urinary tract infections, as well as the risk of bleeding associated with warfarin and nonsteroidal anti-inflammatory drugs (NSAIDs) 3.
• However, it is known that warfarin can interact with various medications, including antibiotics, which may affect its anticoagulant activity 3.
• Nitrofurantoin, like other antibiotics, may have the potential to interact with warfarin, although this specific interaction is not mentioned in the provided studies.
• Patients taking warfarin should be cautious when taking other medications, including antibiotics like nitrofurantoin, and should be monitored closely for any signs of bleeding or changes in their anticoagulation status 3.

Efficacy and Safety of Nitrofurantoin

• Nitrofurantoin has been shown to be effective in treating uncomplicated urinary tract infections, with clinical cure rates ranging from 51% to 94% 2, 5.
• The medication is generally well-tolerated, with mild and reversible side effects, primarily gastrointestinal in nature 2, 5.
• Nitrofurantoin has a low risk of promoting antimicrobial resistance, making it a reasonable alternative to other antibiotics for the treatment of uncomplicated urinary tract infections 4, 6.

Risk of Bleeding with Warfarin

• The use of warfarin can increase the risk of bleeding, particularly when combined with other medications like NSAIDs 3.
• The risk of gastrointestinal bleeding and general bleeding is significantly increased when warfarin is taken with NSAIDs or COX-2 inhibitors 3.
• Patients taking warfarin should be aware of the potential risks and take precautions to minimize their risk of bleeding 3.