Background There is a worrisome increase in opioid prescription worldwide. Their use and overuse may cause adverse outcomes. Aim To determine incidence and characteristics of opioid prescription at discharge at an emergency department (ED). Material and Methods A prospective observational study in a random sample of adult patients attended at an ED of a teaching hospital. We reviewed medical records prescriptions for each patient to collect information about drugs prescribed, reason and medical indication of use (doses and duration). Results A total of 1,001 patients aged 50 ± 20 years (61% women) were studied. Seven percent of patients received an opioid prescription at discharge from the ED, mainly to treat renal and back pain. The dose, duration of treatments or both were incompletely described in 54% of prescriptions. The dose of tramadol in drops was incomplete in 96% of prescriptions. Conclusions Seven percent of patients discharged from an ED received an opioid prescription, mainly to treat non-oncological acute pain. The lack of information detected in the prescriptions affected quality, safety and effectiveness of the treatment, especially when pharmaceutical formulations were drops.