The corticosteroid equivalence calculations are based on old studies comparing the degree of ACTH suppression following a single dose of corticosteroid. The degree of suppression depends on the dose of steroid given as well as the timing of that dose in relation to the drug's half-life. These calculations may not be accurate following repeated doses of steroid.

References
Meikle AW et al. Potency and duration of action of glucocorticoids. Effects of hydrocortisone, prednisone and dexamethasone on human pituitary-adrenal function. Am J of Med 1977;63;200.
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