Neo mercazole used to be used for cats, according to the CP lady (but that was the one that caused her to develop thyroid issues, although she was crushing it). I was surpised at JS's vet being able to prescribe it, I thought they had to use either Felimazole or Vidalta, will ask my vet when Sam's bloods are re-done. I know with Tiger, she had been given a med from a different vet previously that shouldn't be given due to the cascade (it wasn't licenced for use in cats and there was a med that was), and my vet said I could use it at my own risk, and if it worked, then she could apply for it to be used, but she had to prove she had used alternative treatments first and that they hadn't worked (which they didn't) - I dont think it is given that they will give permission though (it didn't work with her, so wasn't taken any further)