Effects on Adrenal Cortex
"The administration of steroids for a period exceeding two weeks runs the risk of suppression of the adrenal cortex. Patients on steroids for less than one week may have them stopped without the risk of adrenal suppression. In people who take large doses of corticosteroids, such as prednisone or prednisolone the function of the adrenal glands will become suppressed. This suppression occurs because these large doses prevent the hypothalamus and pituitary glands from producing the hormones that normally stimulate adrenal function."
Excerpt from cop-boop.org.uk (site no longer active)