It is prescription only. Although it's sold online it still needs a prescription. A tumour of the pituitary can lead to Cushings. The other type of Cushings is from the adrenal glands rather than the pituitary. One type is treatable, the other isn't. I don't think Charlie had it. His fur was amazing. He had the usual old age symptoms, drinking more - and a bit of a belly. He did have some pigmented skin - but there are other things that can cause these and with Cushings there is usually significant fur loss and scabbing sores. Although fur loss doesn't necessarily mean Cushings either. There's a very good article about it but I can't link it as it's an old http website. If you google "Hammy's World - cushings" - it takes you to the website site map and you can find the link to the Cushings info on there. The owner died recently and there was a notice up saying they would maintain the website as it's what he would have wanted.
Charlie had no signs of having a tumour either, it was only latterly I suspected that.