OBJECTIVE: To determine the optimal weekly work schedule for pharmacist’s medication review on potential parenteral-oral drug switching that maximizes the net benefit per intervention. METHODS: Medication charts of patients over one year of age who were prescribed intravenous (IV) drugs in a tertiary care hospital were reviewed prospectively by two clinical pharmacists during a 14-day period in 2003. Candidates for the switching were identified for detailed drugs and dosage regimens used. Net benefits were calculated based on potential savings due to the difference in costs between IV and oral drugs minus the labor cost of pharmacist man-hours. Detailed daily cost information was used to simulate models for five patterns of weekly work schedule. RESULTS: Of a total of 8825 patient-days of medication chart reviews, 37.0% was prescribed for the targeted IV drugs. According to the switching guideline, 308 patients were identified as the candidates for parenteral-oral drug switching. The switching candidates accounted for 878 patient-days of prescribed drugs, which were 26.9% of the cases with IV drugs. The IV antimicrobials were prescribed most frequently (51.0%). The rest was antisecretory drugs, antiepileptics, and steroids. Most switching candidates were admitted to surgery wards (45.1%) and medical wards (26.6%). Potential net cost savings during 14 days of pharmacist’s review accrued to the total of Baht 157,430. The net benefits per month estimated by a simulation of various work schedules were Baht 98,504 for the 7-day a week schedule; Baht 80,454 for the 5-workday schedule; Baht 79,960 for the three-day a week schedule; Baht 59,738 for the twice a week schedule; and Baht 37,008 for the once a week schedule. CONCLUSION: The parenteral-oral drug switching intervention by pharmacist’s prospective medication chart review is a cost-saving program. The net benefit is maximized when the pharmacist works in the patient care wards on a daily basis.