Drug sensitivity in collie-type breeds (MDR1)

Collies and the breeds descended from them can carry a mutation in the MDR1 (ABCB1) gene that removes a pump protecting the brain from certain drugs. The mutation was found in Collies, Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, English Shepherds, McNabs, Longhaired Whippets, Silken Windhounds and a small share of German Shepherds and Border Collies, with three quarters of Collies carrying at least one copy¹. Affected dogs can react severely to ordinary doses of some antiparasitic drugs (ivermectin and related compounds at high doses), the antidiarrhoeal loperamide, some chemotherapy drugs, and some sedatives and anti-nausea drugs; the American Animal Hospital Association summarises the drugs to avoid or reduce and recommends genetic testing for at-risk breeds and their mixes².

For a senior dog this is worth settling once. A cheek-swab test tells you whether your dog carries the mutation, and the result belongs in the notes every clinic sees, because older dogs are prescribed more drugs, not fewer.

This is general information, not a diagnosis. Your vet knows your dog.

Sources

1. Breed distribution and history of canine mdr1-1Δ, a pharmacogenetic mutation that marks the emergence of breeds from the collie lineage (Neff MW et al.). Proceedings of the National Academy of Sciences 101(32):11725-11730, 2004 (peer reviewed). Accessed 2026-09-22.
2. White feet don't treat: Considerations for dogs with MDR1 mutations. American Animal Hospital Association, NEWStat. Accessed 2026-09-22.