Abstract Purpose Formative assessments can help motivate students and ease students’ learning through feedback. There is a pressing need for improvement of clinical pharmacotherapy (CPT) education since junior doctors make many prescribing errors. The aim of this study was to determine whether a formative assessment with personalized narrative feedback helps medical students to increase their prescribing skills. Methods This retrospective cohort study was conducted among masters’ medical students at Erasmus Medical Centre, The Netherlands. Students made a formative and a summative skill-based prescription assessment, both during clerkships as part of their regular curriculum. Errors in both assessments were categorized by type and possible consequence and compared with each other. Results A total of 388 students made 1964 errors in the formative assessment and 1016 in the summative assessment. Most improvements after the formative assessment were seen for mentioning the weight of a child on the prescription (n = 242, 19%). Most new and repeated errors in the summative assessment were missing usage instructions (n = 82, 16% and n = 121, 41%). Conclusions This formative assessment with personalized and individual narrative feedback has helped students to increase the technical correctness of their prescriptions. However, errors repeated after the feedback were predominantly errors showing that only one formative assessment has not yet enhanced the clinical prescribing enough.