Hypoadrenocorticism is treated with fludrocortisone (trade name Florinef)   or a monthly injection of Percorten-V ( desoxycorticosterone pivalate , DOCP) and prednisolone or Zycortal.     Routine blood work is necessary in the initial stages until a maintenance dose is established.  Most of the medications used in the therapy of hypoadrenocorticism cause excessive thirst and urination. It is absolutely vital to provide fresh drinking water for a canine suffering from this disorder. 
Other side effects which your vet will be on the lookout for include the increased risk of infection (due to suppression of the immune system), stomach ulcers, blood clots and diabetes (particularly in cats). In some dogs and cats, determining the appropriate dosage of steroids can be challenging, and it can be difficult to ensure that the right amount of medication is given to control the disease without having significant side effects. Sometimes we need to use other drugs to compliment the use of steroids, allowing us to reduce the steroid dose whilst still controlling the disease.
Steroids killed nine-year-old Lexie McConnell after only five and a half weeks. In August 1993, Lexie was diagnosed as having toxoplasmosis. The consultant put her on 80 mg per day of prednisolone. Immediately, she suffered severe side effects, huge weight gain , terrible pains, holes in her tongue and black stools. After nearly a month, at her parents' pleading, the doctors quickly lowered the dosage to 60 mg, 40 mg, 20 mg. In excruciating pain, Lexie was taken to a hospital, where it was discovered she'd contracted chickenpox. Four days later, she died. A few years later, another eye specialist declared that a simple course of antibiotics could have cleared up her infection. The above excerpt is from Ursula Kelly's site