Background Oral anticoagulant therapy is at high risk of interactions, potentially serious, because of the nature of the drugs and the older patients, with chronic diseases and treated with multiple medications.

Purpose to evaluate the adverse events, the outcome, the suspected and concomitant medical products; to analyse the interactions, already known or not yet reported in literature, with drugs, phytotherapies, homeopathic.

Materials and methods The project involves 7 hospitals for an amount of 11,000 patients treated. The authors analysed the reports collected by Spedali Civili of Brescia, the coordinator centre, for the period 01/11/2009 – 30/09/2011. Patients with a suspected ADR (Adverse Drug Reaction) answered to a questionnaire during the visit or by phone; the collected data were put into a provided database, and also into the National Network.

Results 266 reports were recorded. The 86% concerned patients between 60 and 89 years, male for 59.8%. The most frequent ADRs are changes of INR, increased in 21.8% (INR > 6 in 1.5 %) decreased in 15%. 71 patients (26.7%) developed major haemorrhage. 56% of the reported cases are not serious, while 40.2% required hospitalisation and / or lengthening of hospitalisation, 194 patients (72.9%) resolved completely the ADR, 45 (16.9%) improved, for 16 the reaction was unchanged or worsened and 6 died. The most frequent indication for which anticoagulant was prescribed was atrial fibrillation. 16 patients took 10 concomitant medications; in 9 cases supplements, or herbal products could be correlated to adverse reaction. The medium hospitalisation length after an ADR was 7.5 days.

Conclusions despite warfarin and acenocoumarol are used for a long time, they are still responsible for serious adverse reactions. It's important to report the ADRs, especially those with new drugs and non-conventional drugs in order to identify the fatal interactions to avoid.