This study shows the cost of drug treatment failure, illustrating the need for initial treatment success with proper drug choice and compliance. OBJECTIVE: This study examines the cost of prescription drugs used in the treatment of sinusitis and the cost of treatment failure. Economic costs are strong incentives to treat a disease effectively, reducing relapses and antibiotic resistance. METHODS: The Idaho Medicaid database was used to analyze drug costs. The data was divided into 2 groups, a less than 15 but more than one year old group labeled Group 1. Group 2 consisted of patients 15 or more than 15 years old. The treatment was considered a failure when a patient returned within 30 days of a physician visit. Thus all treatment failures could be captured. RESULTS: There were 15, 568 patients (9.8%) who had at least one diagnosis of sinusitis, comprising of 36.2% males and 63.8% females. The total cost of prescription drugs was $529,065. For Group 1, the total cost of drugs for the first episode was $182,499 written for 6,594 patients. Of these 450 patients had a treatment failure, costing $17, 365, of which Amoxicillin Tr/Pot. Clav. accounted for 31% of drug cost. For Group 2, the total cost of drugs for the first episode was $264,030 written for 7,107 patients. Of these 592 patients had a treatment failure, costing $37,183. Amoxicillin Tr/Pot. Clav. accounted for 22.3% of drug cost for treatment failure. CONCLUSION: The results show that more expensive drugs are used for treatment failure. This is an incentive for ensuring that a treatment succeeds with proper selection and compliance of drug therapy.