Zemplar is used to treat and prevent secondary hyperparathyroidism (overactive parathyroid glands) in patients with chronic kidney disease. Hyperparathyroidism is a silent condition (causing no symptoms) caused when kidney function declines. Secondary hyperparathyroidism is very common in people in Stages 4 and 5 of chronic kidney disease and can cause the parathyroid glands to become permanently enlarged. Zemplar works by preventing the production of too much parathyroid hormone.