Background Prescribing for the elderly is challenging. A previous observational study conducted in our geriatric psychiatry admission unit (GPAU) using STOPP/START criteria showed a high number of potentially inappropriate drug prescriptions (PIDPs). A clinical pharmacist was added to our GPAU as a strategy to reduce PIDPs. Objective The objective of the present study was to assess the impact of a clinical pharmacist on PIDPs by measuring acceptance rates of pharmacist interventions (PhIs). Setting This study was conducted at the GPAU of Lausanne University Hospital. Method The clinical pharmacist attended four GPAU meetings weekly. Complete medication reviews were performed daily. The clinical pharmacist conducted standard analyses based on clinical judgment and STOPP/START criteria assessment. A PhI was generated when a PIDP was detected. When a PhI was accepted, the PIDP was considered as eliminated. Acceptance rate of PhI was calculated (number of PhI accepted/total number of PhI). Main outcome measure PhIs acceptance rates. Results In a cohort of 102 patients seen between July 2013 and February 2014, a total of 697 PhIs (average 6.8/patient) were made based on standard evaluation (n = 479) and STOPP/START criteria (n = 243). The global acceptance rate was 68% (standard, 78%; STOPP/START, 47%). Conclusion Good PhIs acceptance rates demonstrated that a clinical pharmacist can reduce PIDPs in a GPAU. PhIs based on standard evaluation had a higher acceptance than those based on STOPP/START criteria, probably because they are better adapted to individual patients. However, these two evaluation approaches can be used in a complementary manner.