Sydney has been on hyperthyroidism medicine for over a year and a half. We actually had to up her dosage to one whole pill twice a day - in order to get her levels under control. I have been taking her in monthly for blood tests to check her kidney function and her levels. Just yesterday I brought her in and her levels were still within the normal range. My vet seemed to indicate that if your cat already has a pre-existing kidney issue, then yes - this medication can damange the kidney's beyond repair. However, if your cat has normal functioing kidneys, then they can live on the medication well past 6 months. Hope this helps some.