Hi, no expert here but for what its worth, the few horses I have had in my care were lethargic already with the untreated Cushing's. The higher the ACTH the more lethargic they were. One did OK on Prascend the other had to start @ 1/4 tab and stay there for a long time ( a month perhaps ) before she could go up because she would stop eating. I have a lot more experience in human medicine and there are many people that have a hard time adapting to a an Rx that does eventually help, they just have to go @ their own pace not the typical. Hope that helps and your vet can support you well.
lm