Aims: Prescribing medication is a complex process that, when done inappropriately, can lead to adverse drug events, resulting in patient harm and hospital admissions. Worldwide cost is estimated at 42 billion USD each year. Despite several efforts in the past years, medication-related harm has not declined. The aim was to determine whether a prescriber-focussed participatory action intervention, initiated by a multidisciplinary pharmacotherapy team, is able to reduce the number of in-hospital prescriptions containing ≥1 prescribing error (PE), by identifying and reducing challenges in appropriate prescribing. Methods: A prospective single-centre before- and after study was conducted in an academic hospital in the Netherlands. Twelve clinical wards (medical, surgical, mixed and paediatric) were recruited. Results: Overall, 321 patients with a total of 2978 prescriptions at baseline were compared with 201 patients with 2438 prescriptions postintervention. Of these, m456 prescriptions contained ≥1 PE (15.3%) at baseline and 357 prescriptions contained ≥1 PEs (14.6%) postintervention. PEs were determined in multidisciplinary consensus. On some study wards, a trend toward a decreasing number of PEs was observed. The intervention was associated with a nonsignificant difference in PEs (incidence rate ratio 0.96, 95% confidence interval 0.83–1.10), which was unaltered after correction. The most important identified challenges were insufficient knowledge beyond own expertise, unawareness of guidelines and a heavy workload. Conclusion: The tailored interventions developed with and implemented by stakeholders led to a statistically nonsignificant reduction in inappropriate in-hospital prescribing after a 6-month intervention period. Our prescriber-focussed participatory action intervention identified challenges in appropriate in-hospital prescribing on prescriber- and organizational level.