My Cushing Syndrome horse.
I strongly suggest you speak with your veterinarian about your horse and the effects of pergolide.
In very general terms, pergolide is what is known as a "Dopamine agonist," meaning it encourages the brain to make dopamine. Dopamine targets cells in the pars intermedia section of the horse's pituitary gland, and when dopamine input is lost as a result of equine Cushing's syndrome, these cells become overactive and the pars intermedia becomes enlarged. Pergolide works to increase dopamine and reduce the over-activity of these cells, helping to normalize hormone production, including ACTH and cortisol.
A common side effect of pergolide is a decrease or loss of appetite, which is usually temporary. Your veterinarian can help guide you through a proper dosing protocol to help reduce or avoid side effects.
According to Boehringer Ingelheim, manufacturer of Prascend pergolide tablets for horses, improvement of clinical signs of equine Cushing's syndrome was evident within 3 months under proper veterinary care (https://www.prascend.com/treatment_efficacy.html)
I definitely recommend you work with your veterinarian to establish an appropriate treatment protocol for your horse, and to talk through any additional questions you may have. Each horse is different, and not all react the same way to a given treatment, so keep an eye on your horse and keep your vet informed. Good luck!