Introduction: Drug interactions can cause many clinical problems, particularly when the drugs are administered in combination with anticancer agents.
Case report: A patient required two hospitalizations due to risk of bleeding with altered INR probably due to an interaction between gefitinib and acenocoumarol, which resulted in the potentiation of the effect of the latter and acenocoumarol dose adjustment was needed. A causality assessment between the drug-drug interaction and the augmented INR was conducted according to Naranjo algorithm and was classified as a definite adverse drug reaction.
Conclusions: Patient's management recommended is to closely monitor for changes in the effects of coumarin derivatives, if administered concomitantly with antineoplasic agents.