Hydrochlorothiazide is not effective in patients with a creatinine clearance of less than 30 mL per minute. They should not be used in patients with severe renal impairment or anuria. It is also contraindicated to patients with Addison's disease. Thiazide can reduce urinary excretion of calcium, sometimes resulting in mild hypercalcemia and is therefore contraindicated to patients with pre-existing hypercalcemia.