ObjectiveJunior doctors have the highest rates of prescribing errors, yet no study has set out to understand the differences between completely novice prescribers (Foundation year one (FY1) doctors) and those who have gained some experience (Foundation year two (FY2) doctors). The objective of this study was to uncover the causes of prescribing errors made by FY2 doctors and compare them with previously collected data of the causes of errors made by FY1 doctors.DesignQualitative interviews, using the critical incident technique, conducted with 19 FY2 doctors on the causes of their prescribing errors and compared with interviews previously conducted with 30 FY1 doctors. Data were analysed using a constant comparison approach after categorisation of the data using Reason’s model of accident causation and the London protocol.ResultsCommon contributory factors in both FY1 and FY2 doctors’ prescribing errors included working on call, tiredness and complex patients. Yet, important differences were revealed in terms of application of prescribing knowledge, with FY1 doctors lacking knowledge and FY2 misplacing their knowledge. Due to the rotation of foundation doctors, both groups are faced with novel prescribing contexts, yet the previous experience that FY2 doctors gained led to misplaced confidence when caution would have been expedient.ConclusionsDifferences in the contributory factors of prescribing errors should be taken into account when designing interventions to improve the prescribing of foundation doctors. Furthermore, careful consideration should be taken when inferring expertise in FY2 doctors, who are likely to prescribe in contexts in which their experience is little different to an FY1 doctor.