The % of the patients had a prothrombin time (INR) in the therapeutic range; % had minor complications; % of the patients complied with the criteria of multiple medication and % complied with the criteria of multiple medication in the last 6 months. The drugs used with a higher capacity to interact are: anti-ulcer (%), locomotor system (%), cardiovascular drugs (%), lipid lowering drugs (%), and antidiabetics (%). The number of drugs used in the last 6 months is associated with the presence of haemorrhagic complications (odds ratio [OR], ). Allopurinol and pantoprazole had a significant relationship with the presence of minor haemorrhages (OR, and , respectively). The variables associated with the presence of a haemorrhage were: allopurinol (OR, ), number of controls with an INR outside the therapeutic range in the last 6 months (OR, ) and time on treatment (OR, ).