Before Interceptor mine used to get Heartgard even though it was supposedly reformulated and safe for shelties. It was, basically, until we had to up the dosage for Marcus who gained enough weight to put him in the next higher weight group. This was 3 years ago. He had a near fatal toxic reaction and we almost lost him. The vets thought I was nuts when I said it was an Ivemectrin reaction and my vet even called the manufacturer since they had never seen a toxic reaction after the reformulation. Heartgard said Marcus was the first case since they reformulated it to report a toxic reaction. After that all mine were switched to Interceptor and have been fine since. They have no issues with taking it since mine would eat anything