BACKGROUND: We aimed to compare serum lactate levels of multi-drug poisoned patients to determine whether knowing the level may help emergency clinicians in predicting the patients’ prognoses. METHODS: The patients were divided into two groups according to the number of kinds of drugs taken (Group 1: patients took 2 kinds of drugs; Group 2: patients took 3 or more kinds of drugs). The groups’ initial venous lactate levels, lactate levels before discharge, lengths of stay in the emergency department, hospitalisation units, clinics, and outcomes were recorded on the study form. These findings of the patient groups were then compared. RESULTS: When we evaluated the first lactate levels and lengths of stay in the emergency department, we found that 72% of the patients with initial lactate levels ≥13.5 mg/dL stayed more than 12 hours in the emergency department. Twenty-five (30.86%) patients in the second group stayed ≥12 hours in the emergency department, and their mean initial serum lactate level was significantly related (p=0.02, AUC=071). The mean initial serum lactate levels of both groups were positively related with their lengths of stay in the emergency department. The mean initial lactate levels of patients who stayed ≥12 hours and those who stayed