Ivermectin dosage for a large / giant breed

Old English Sheepdog — MDR1 (ABCB1) mutation risk — genetic testing required before ivermectin

This breed carries a meaningful frequency of the MDR1 gene mutation, which prevents the drug-efflux protein at the blood-brain barrier from clearing ivermectin out of the central nervous system. Affected dogs develop neurotoxicity at doses other dogs tolerate without difficulty, and at the investigational oncology doses discussed on this site that reaction can be fatal. A cheek-swab test through Washington State University or a commercial laboratory settles the question and must be done before the first dose.

⚠ No canine dose-finding study supports this dose range

The 0.3–0.6 mg/kg figure circulates widely in owner protocols and is reproduced here because readers will encounter it elsewhere. It is not established by any canine study. The published canine work tested ivermectin on mammary tumour cells in culture and grew its xenografts in mice, so it establishes a mechanism rather than a dose a dog was shown to tolerate. For scale, canine heartworm prevention uses roughly 0.006 mg/kg — this range is fifty to a hundred times that. Treat it as a description of what people do, not as a validated dose, and set any actual dose with a veterinary oncologist.

⚠ MDR1 gene mutation — testing is mandatory

Dogs carrying the MDR1 (ABCB1) mutation cannot clear ivermectin from the brain and suffer neurotoxicity at doses other dogs tolerate. At the investigational oncology doses discussed here, that reaction can be fatal. Affected breeds include Collies, Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, Border Collies, Long-haired Whippets, and any mixed breed with herding ancestry. The test is a simple cheek swab available through Washington State University and commercial laboratories. It must be done before the first dose.