Hi, Joan - I just want to chime in and agree with what Lorna and Maggie have said. Sonny is so very clearly PPID (Cushing's) that taking him off pergolide would be exactly the same as removing insulin injections from a Type 1 Diabetic, or preventing a clinically depressed person from having their anti-depressants. The outcome could not possibly be good. Pergolide side-effects in the horse are pretty minimal, and usually restricted to the first week or so (lethargy, lack of appetite, etc). The heart-valve dysfunction that happened to some humans on pergolide has been shown to be dose-dependent, and even horses at the highest doses in our data base don't come close to the mg per kg body weight that some humans were taking.
The side effects of PPID, on the other hand, are very well documented, and can be truly terrible, including persistent infection (due to suppressed immune system); laminitis; and feeling lousy. There is muscle loss, weight loss, and weakness. Some of them show depressed appetites. There have been a small number of horses seen on this list who died from infection after the owner stopped using pergolide because of an inaccurate perception that using drugs was bad, or that pergolide might be "toxic".
BI, that manufactures Prascend, has had a very aggressive marketing scheme aimed at vets, including letters saying that compounded pergolide is illegal, unethical, and much like boiling babies. Their marketing is impressive, I will say. However, compounding pharmacies that have butted heads with the FDA have won. If your vet is concerned, all he or she needs to do is write a script for "pergolide", instead of for "Prascend". You then take it to the pharmacy, or the vet's office faxes it to the pharmacy, and the vet is totally in the clear. If the vet's office were to try to sell compounded pergolide, that would definitely be illegal (for a variety of reasons). However, providing a script is perfectly safe.
Hang in there.
Jaini Clougher (BSc,BVSc)
Merlin (over the bridge) ,Maggie,Gypsy, Ranger