Great article on how much of a dose it takes to be toxic to a dog, depending on their size. If you happen to own a Collie, Sheltie, Aussie, Border Collie,  the dose may be 1/3 the dose calculated in this article.

netpetmagazine.com.

This sensitivity of the heart to some chemicals is because the  members of the “Herding Breeds” may  inherit a Multi-Drug Sensitivity gene called  MDR1.  Washington State University, Veterinary Pathology laboratory has an inexpensive DNA test that can be done by owners to determine if your dog has any copies of the MDR1 gene. This information is important for planning on dosage of several drugs i.e.; heartworm preventatives, some antibiotics and chemicals that affect the heart, chemotherapy plus some anesthestics and tranquilzers. Simply knowing if your dog has the gene, allows your Vet to adjust the dose of the candidate drugs or choose another option when available. Follow the link below for more info:

Breeds of dogs affected with the mutant MDR1 gene. Information from the VCPL at Washington State University..