I can't say it any better in my own words...so this is a cut and paste from wikipedia
I know I have taken it in my past for asthma, and so did my grandmother. It was rough on me, and simply tore my grandma up.
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Dependency
Adrenal suppression will occur if prednisone is taken for longer than 7 days. This will cause the body to lose the ability to synthesize natural corticosteroids, resulting in dependence on prednisone. For this reason, prednisone should not be abruptly stopped if taken for more than seven days, and instead, the dosage should be gradually reduced. This weaning process may be over a few days if the course of prednisone was short, but may take weeks or months if the patient had been on long-term treatment. Abrupt withdrawal may lead to an Addisonian crisis. For those on chronic therapy, alternate-day dosing may preserve adrenal function, thereby reducing side-effects.[4]
Glucocorticoids act to feedback inhibit both the hypothalamus (decreasing Corticotropin-releasing hormone [CRH]) and corticotrophs in the anterior pituitary gland (decreasing the amount of Adrenocorticotropic hormone [ACTH]). For this reason exogenous glucocorticoid analogues down-regulate the body's ability to naturally produce glucocorticoids. This mechanism leads to dependence in a short time and can be very dangerous if medications are withdrawn too quickly. The body must have time to begin synthesis of CRH and ACTH and for the adrenal glands to begin functioning normally again.
Side Effects:
Short-term side-effects, as with all glucocorticoids, include high blood glucose levels, especially in patients who already have diabetes mellitus or are on other medications that increase blood glucose (such as tacrolimus), and mineralocorticoid effects such as fluid retention (it is worth noting, however, that the mineralocorticoid effects of prednisone are very minor; this is why it is not used in the management of adrenal insufficiency unless a more potent mineralocorticoid is administered concomitantly).
Additional short-term side-effects include insomnia, euphoria, and, rarely, mania (particularly in those suffering from Bipolar I and II).
Long-term side-effects include Cushing's syndrome, truncal weight gain, osteoporosis, glaucoma, type II diabetes mellitus, and depression upon withdrawal