http://en.wikipedia.org/wiki/Prednisone

Adrenal suppression occurs if prednisone is taken for longer than 7 days, a condition which means the body is unable to synthesise natural corticosteroids and becomes dependent on the prednisone taken by the patient. For this reason, prednisone should not be stopped abruptly if taken for longer than seven days, but needs to be reduced slowly; this reduction may be over a few days if the course of prednisolone was short, but may take weeks or months if the patient has been on long-term treatment. Abrupt withdrawal will lead to an Addisonian crisis, which may be life-threatening.