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Multidrug Resistance

MDR1 FAQs - Australian Shepherd Health & Genetics Institute

ASHGI DOES NOT recommend removing dogs from the breeding pool solely because they carry one or even two copies of the MDR1 mutation. This mutation is only a problem because we administer drugs to dogs to which they can react.


MDR stands for Multidrug Resistance and the mutation in the MDR1 gene prevents many breeds, including Australian Shepherds, from removing many drugs from the brain resulting in toxicity. The MDR1 mutation is believed to occur in approximately 50% of Australian Shepherds. Dogs with the MDR1 mutation can be 200 times more sensitive to the drugs than dogs that do not have this mutation.


How common is the MDR1 mutation in Aussies? One in two Aussies has at least one copy of the gene.

Dogs with the mutation lack a protein (P-glycoprotein) which is responsible for pumping the drugs from the brain. When this process is interrupted the drugs build up and permeate the brain and neurological toxicity results.

Since there is such a wide range of symptoms and so many different drugs that can affect dogs with the MDR1 gene mutation it is best to, of course, avoid use of the implicated drugs and to keep a close watch on your dogs for any reaction after administering any medication no matter how seemingly harmless. Remember, Imodium (loperamide) is used for the treatment of diarrhea is on the list of drugs that are considered dangerous for dogs with the MDR1 sensitivity.

The MDR1 Mutation Can Cause Sensitivity To Many Medications

The best way to keep track of what drugs are on the MDR1 list is to look up the list of current drugs on the Washington State University website:

Problem medications for dogs | Program in Individualized Medicine | Washington State University (

Trifexis, Ivermectin For Dogs

While you will want to investigate any medication you give your dog, Trifexis (milbemycin oxime + spinosad) and Ivermectin are those most frequently mentioned in our forums as being of particular concern, no doubt due to their wide availability and use as anti-parasitics.

While Ivermectin for dogs is not recommended for dogs that have the MDR1 gene mutation there is also growing concern about other popular medications like Trifexis. Whether the warnings are based on hard scientific evidence or anecdotal "evidence" I would have to recommend erring on the side of caution.


Dog Breeds Affected by MDR1 Gene Mutation, Ivermectin for Dogs, Etc.

  • Australian Shepherd

  • Mini Australian Shepherd

  • Collie

  • Old English Sheepdog

  • English Shepherd

  • Shetland Sheepdog (Sheltie)

  • German Shepherd

  • Longhaired Whippet

  • Silken Windhound

  • Border Collie

  • McNab

  • Dogs of mixed breeds that include herding breeds

While Ivermectin for dogs or Trifexis may grab the spotlight there are many drugs that can be a direct threat to your dog. There are also other drugs that can cause your dog to be vulnerable even if they do not test as carriers or as positive for the MDR1 gene mutation.

You will also want to be vigilant about potential sources of these drugs. While you may not give them to your dog directly, what about other animals (either yours or a neighbor's) that are being treated? Is your neighbor treating their horses with ivermectin? 

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