Heavy antibiotic users are those individuals with the highest exposure to antibiotics. They play an important role as contributors to the increasing risk of antimicrobial resistance. We applied different methods to identify and characterise the group of heavy antibiotic users in Spain as well as their exposure to antibiotics. Data on outpatient prescribing of antimicrobials (ATC J01) in 2010 were obtained from a prescription database covering Aragón (northeastern Spain). The antimicrobial consumption at the individual level was analysed both according to the volume of DDD and the number of packages purchased per year. Heavy antibiotic users were identified according to Lorenz curves and characterised by age, gender and their antimicrobial prescription profile. Lorenz curves demonstrated substantial differences in the individual use of antimicrobials. Heavy antibiotic users (5% of individuals with highest consumption) were responsible for 21% of the total DDD consumed and received ≥6 packages per year. Elderly adults (≥60 years) and small children (0-9 years) were those exposed to the highest volume of antibiotics and with the most frequent exposure, respectively. Heavy users received a high proportion of antibiotics not recommended as first choice in primary health care. In conclusion, heavy antibiotic users consisted mainly of children and old adults. Inappropriate overuse of antibiotics (high quantity, high frequency and inappropriate antibiotic choice) lead to a substantial risk for the emergence and spread of resistant bacteria and interventions to reduce overuse of antibiotics should therefore primarily be targeted children and elderly people. This article is protected by copyright. All rights reserved.