Effects of multiple intramuscular injections and doses of dexamethasone on plasma cortisol concentrations and adrenal responses to ACTH in horses.

Adrenocortical function was assessed in horses given multiple IM doses of dexamethasone to determine the duration of adrenocortical suppression and insufficiency caused by 2 commonly used dosages of dexamethasone (0.044 and 0.088 mg/kg of body weight). Dexamethasone was administered at 5-day intervals for a total of 6 injections. Daily blood samples were collected. The plasma was frozen and later assayed for cortisol. An ACTH response test was determined 2 days before the first injection of dexamethasone and again 8 days after the last dexamethasone injection. Maximum suppression of plasma cortisol was observed in horses given both dosages of dexamethasone (0.044 and 0.088 mg/kg). Plasma cortisol concentrations returned to base-line values in all horses by 4 days after dexamethasone injection. Normal ACTH responses observed after 6 dexamethasone injections given at 5-day intervals indicated that measurable adrenal atrophy did not develop under the conditions of this study.