To investigate factors that predict the probability and duration of mechanical ventilation in quetiapine overdose, and if cardiac toxicity occurs, this cohort study involved 176 patients presenting to a toxicology unit on 286 occasions with quetiapine overdose. Patient demographics, dose, coingestants, single dose activated charcoal (SDAC) administration, requirement for and duration of mechanical ventilation and electrocardiogram parameters (heart rate, QT, QRS) were obtained. A fully Bayesian approach using logistic regression and time-to-event analysis was undertaken to investigate the relationship between predictor variables and the requirement for and duration of intubation. QT versus heart rate was plotted on a QT nomogram to investigate QT prolongation. The commonest clinical effects were central nervous system depression on 136 occasions (48%) and tachycardia (67%). There were no malignant arrhythmias and an abnormal QT occurred in only 24 admissions (8.4%), all with tachycardia. Hypotension (systolic blood pressure <90 mmHg) occurred on 35 occasions (12%). The logistic regression model supported dose and SDAC (<2 h) influencing the probability of intubation, but not age, sex, therapeutic use of quetiapine or coingestants. The probability of intubation was 10% after 2 g, 22% after 5 g, 37% after 10 g and 55% after 20 g and SDAC resulted in a reduced probability of intubation of 7% for 2 g ingestion. The median duration of ventilation was 22 h (interquartile: 19-28 h), which was not affected by SDAC. Ingested dose can inform early decision making about requirements for intensive care unit admission and intubation. SDAC seems to have only modest effects on outcomes but may be considered within 2 h for large ingestions. Electrocardiogram monitoring is unlikely to be necessary.