We have developed a computerized checking system for drug interactions as a subsystem of the dispensing support system linked to the prescription order entry system in Oita Medical University Hospital. Using this system, interactions between two drugs in separate prescriptions can be checked for overlapping administration periods. The outline of the detected interactions can be printed on the prescription. The details (cause, effect, management, reference and so on) of them can be displayed on the computer screen. Two hundred and twenty-nine cases of unfavorable drug interaction have been detected after 3 months of using this system. A revision of the prescription was performed in 20.5% of them, according to the advice from clinical pharmacists.