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Possible interaction between ciprofloxacin and warfarin
1Pharmacy Service, Veterans Administration Medical Center, Denver, CO.
Abstract:
A 72-year-old man had been taking warfarin for a pulmonary embolus and recurrent deep-vein thromboses. His prothrombin times (PTs) were maintained between 15 and 18 sec (PT ratio 1.25-1.5 x control) for several months on a dose of warfarin 2.5 mg/d. Six weeks prior to starting ciprofloxacin 500 mg bid, the patient's PT was 15.5 sec (Pt ratio 1.29 x control). After one week of ciprofloxacin, his PT had increased to 22 sec (PT ratio 1.83 x control). No other causes for the increase were apparent. It is recommended that patients receiving both medications have their prothrombin times carefully monitored and warfarin doses adjusted only as necessary.
Insights
Ciprofloxacin significantly increased warfarin
Area of Science:
- Pharmacology
- Clinical Medicine
Background:
- Warfarin is a common anticoagulant for venous thromboembolism.
- Maintaining therapeutic anticoagulation requires regular monitoring of prothrombin time (PT).
Observation:
- A patient on stable warfarin therapy (2.5 mg/d) experienced elevated PT levels after initiating ciprofloxacin (500 mg bid).
- The patient's PT increased from 15.5 sec (1.29 x control) to 22 sec (1.83 x control) within one week of ciprofloxacin use.
Findings:
- Ciprofloxacin significantly potentiated the anticoagulant effect of warfarin.
- No other confounding factors were identified for the observed increase in PT.
Implications:
- Concurrent use of ciprofloxacin and warfarin necessitates close prothrombin time monitoring.
- Clinicians should be prepared to adjust warfarin dosage when these medications are co-administered.