We examined the frequency of dangerous drug combinations in common antihypertensive medication regimens used for outpatients at Asahikawa Medical University Hospital and those under the State of Iowa Medicaid program.We also investigated the possibility of reducing the frequency through a computerized checking system for drug interactions at Asahikawa Medical University Hospital.We noted that 3,501 of the combinations were mentioned in the“Drug Interaction Facts”book as interacting combinations of antihypertensive drugs,and contraindicated and inappropriate combinations(importance level 1~2)accounted for 30.7%(1,074 combinations)of them.The majority of interactions for the contraindicated combinations were cardiovascular disorder-related.For the Medicaid patients,there was a strong correlation(r=0.741)between age over 30 years and frequency of importance level 1~2 in regimens and 21.4% of all combinations were importance level 1~2.At 0.4 times per prescription,the frequency was highest for potassium-sparing diuretics.On the other hand,at Asahikawa Medical University Hospital,the frequency of importance level 1~2 did not depend on age,but accounted for 30% of the all combinations whether the checking system was in operation or not.Further,at 0.5 times per prescription,the frequency for potassium-sparing diuretics was higher than that for the Medicaid patients.Thus the use of the checking system was not effective in reducing the frequency.We therefore considered that the criteria used for evaluating interacting drug pairs in Japan(refer to attached document)are different from those used in the United States.Our findings also suggest that the use of clear criteria for evaluating drug interactions is very important in preventing dangerous drug combinations for common medications.