Corticosteroids are the most widely used immunosuppressants in dogs and cats. Two drugs are frequently used: prednisone (or prednisolone), and dexamethasone. Prednisone is used for both induction and maintenance, whereas dexamethasone is typically used in the induction phase, and only brieftly. When dosing these agents, the clinician should bear in mind their relative potency: dexamethasone is seven to eight times more potent than prednisone.

Corticosteroids act by three major mechanisms: they suppress mononuclear-phagocytic activity (immediate effect); they cause elution of the antibody molecules from the surface of the target cells (immediate effect); and they suppress the production of immunoglobulins (delayed effect). In addition, they appear to impair nutrophil bacterial killing ability and cell-mediated immunity.

During the induction phase of immunosuppression, prednisone (or equivalent doses of dexamethasone) is administered daily for 7 to 10 days in doses of approximately 4 mg/kg in dogs and 4 to 8 mg/kg in cats. After this, the dose is decreased and the interval between administration is lengthened to every other day to prevent interference with the hypothalamic-pituitary-adrenal axis. Once the disease is in remission, the dose of corticosteroids, as well as of other immunosuppressants, should be decreased gradually to prevent sudden relapses. Prednisone and prednisolone are considerably safer than dexamethasone for long-term treatment. Because in cats the liver is not effective at methylating, prednisolone (methylated prednisone) may be a better choice than prednisone.

Adverse effects of corticosteroid treatment include iatrogenic hyperadrenocorticism, gastrointestinal tract ulceration, recurrent urinary tract infections, and pancreatitis. Acute gastrointestinal tract ulceration or pancreatitis is more common in dogs receiving dexamethasone than in those receiving prednisone. Indeed, gastroduodenal ulcers or severe acute pancreatitis have been observed in dogs within 24 to 48 hours of administering one dose of dexamethasone. There are minimal or no adverse effects in most cats, although diabetes mellitus (usually transient) may occur.

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