Inhibition by Rifampin of the Anticoagulant Effect of Phenprocoumon

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Abstract
THE strong antagonistic effect of rifampin on orally given anticoagulants is only indirectly referred to in the English literature.1 We present two cases that illustrate this effect. Report of Cases Case 1.— A 75-year-old man had been receiving long-term phenprocoumon (Marcoumar [Great Britain]; Liquamar, comparable US product) treatment (3 mg daily) for myocardial infarction since August 1970. In November 1971, treatment with rifampin, 600 mg, and isoniazid, 300 mg daily, for a suspected tuberculous spondylitis was begun. With the addition of these drugs, the patient's average phenprocoumon requirement increased to 6 mg/day. One year later after a severe gross hematuria, it was shown that his blood was incoagulable. Ten milligrams of phytonadione (Konakion) were given orally. The bleeding stopped, and therapeutic clotting values were restored and maintained with an average phenprocoumon dosage of 3 mg daily. On inquiry, it appeared that rifampin therapy had been discontinued three months before

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