Alcohol is the most widely consumed toxic in Spain. Excessive alcohol intake is responsible for a significant number of visits to emergency departments (EDs), and what occurs may not only have severe medical consequences, but also serious legal implications. Most Spanish EDs lack specific protocols concerning the correct determination of blood alcohol content (BAC). The present work aims to review the technical, ethical and legal problems surrounding this test. Since ethanol is metabolized in peripheral blood, blood extraction should be standardized in order to preserve the proper chain of custody. An emergency test for BAC should be performed in two scenarios: patients with an altered level of consciousness of unknown origin (when health-care professionals act for the good of the patient), and situations which may be penalized by law (when health-care professionals act for the good of the community). The latter would include traffic controls and traffic accidents, job-related accidents, criminal activities or harmful domestic behaviour. Health-care professionals are responsible for treating patients' clinical information with due respect and confidentiality. However, professional secrecy may be overridden by legal imperative in certain situations. It is necessary to promote conscious ethical decision-taking by the health-care professional, so that this responsibility does not solely depend on the juridical context.