Aims To determine the extent of inappropriate prescribing of combination diuretics and the cost implications of protocol implementation. Methods Pharmacist-run medication review clinics in two general practices in Bradford, UK. Results Sixty-one patients, mean ± s.d. age 75.8 ± 12.0 years, were reviewed. Thirty-six met protocol criteria; 30 (49.2%) patients had their potassium-sparing diuretics (PSDs) discontinued. Mean (95% confidence interval) reduction in serum potassium concentrations after intervention in these 30 patients was 0.26 (0.09, 0.43) mmol l-1 (P < 0.01). Twenty-eight (93.3%) patients were within the reference range at follow-up; none was below. Conclusions Of prescribed PSDs, 59% were found to be unnecessary. Using the developed protocol potentially improves drug safety and enables cost savings.