Medicine administration involves patients’ decision-making and cognitive load. In this sense, Patient Information Leaflet (PIL) is the main document for users as it conveys information into tasks. However, most studies have looked at legibility and readability aspects, neglecting the usability of PILs. This paper discusses the results of a study on the effects of graphic presentation of PILs on the simulated use of five medicines, conducted with 60 participants in Brazil, and focusing on their information processing, action and verification errors. The results showed that drawbacks in the graphic presentation of the PILs produced task errors, particularly of dosage verification and manipulation of medicine components. The main conclusion was that the graphic presentation of the Brazilian PILs affected their usability in medicine usage; and therefore may lead treatment of diseases to failure.