Acenocoumarol is the most widely prescribed vitamin K antagonist (VKA) to prevent and treat thromboembolic disorders. It keeps a major place in many indications despite the introduction of new direct oral anticoagulants (DOACs). However, a narrow therapeutic range, an intra-individual variability and drug interactions may lead to serious adverse drug reactions.  Sometimes, a genetic or acquired resistance to this drug may lead to a risky situation. Hopefully, resistance to acenocoumarol is a very rare phenomen.

Here in we present an unsusual case of a suspected resistance to acenocoumarol. This case was notified to the Tunisian National Center of Pharmacovigilance on October 2017 and registered under the number 2449/2017.

A 67-year-old patient with hypertension, diabetes, and coronary disease was treated with captopril, atenolol, atorvastatin, and salicylic acid. In 2017, acenocoumarol treatment was introduced. At biological control, the prothrombin time (PT) was 100%. The doses of acenocoumarol were raised progressively with iterative controls of PT. PT was always 100% even when acenocoumarol reached the dose of 2 g/day. A resistance to acenocoumarol was suspected. The patient was referred to pharmacovigilance department for case analysis. During the patient interview, we discover that the patient was confusing acenocoumarol with atenolol. In fact, when his doctor was increasing the doses of acenocoumarol, the patient increased her intake of atenolol believing that it was acenocoumarol.

A resistance to acenocoumarol was eliminated in this patient since she had never taken the drug. We highlight through this case the importance of patient’s interview. Explaining the indications and the potential adverse events of the drug to patients taking VKA is crucial to ensure a better efficiency of treatment without increasing the risk of bleeding complication.