What is the MDR1 Gene Mutation?

The MDR1 gene mutation is a disorder that inhibits the ability of many herding dog breeds to remove certain drugs from the brain. It can lead up to a buildup of toxins in the brain, which can cause adverse reactions to some common drugs, including seizures, tremors, disorientation, blindness, lack of muscle control or even death.

It’s important to note that the flea, tick, and heartworm medications that we recommend and offer (at DAMC) are at levels recognized as safe for MDR1 susceptible dogs.

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The following dog breeds are affected by MDR1 gene mutation:

  • Australian Shepherd
  • Border Collie
  • Collie
  • English Shepherd
  • German Shepherd
  • Long-Haired Whippet
  • McNab
  • Miniature Australian Shepherd
  • Mixed Breed
  • Old English Sheepdog
  • Shetland Sheepdog
  • Silken Windhound

The dogs listed above may also be affected by:

  • Antibiotics such as erythromycin and rifampin
  • Anti-cancer drugs such as doxorubicin, vinblastine and vincristine
  • Anti-diarrheal drugs such as loperamide (Imodium®)
  • Certain parasite-control products such as ivermectin (in high doses), milbemycin, moxidectin and selamectin
  • Pain medications such as butorphanol
  • Tranquilizers/sedatives such as acepromazine

It’s important to note that when given at the recommended dose, the medications most often recommended by veterinarians, such as monthly heartworm and flea preventives offered by Bravecto, are at levels safe enough even for dogs with the MDR1 mutation.

Your dog should be tested for the gene before it receives any of the drugs listed above. The results can help you and your veterinarian when planning your pet’s future healthcare. An MDR1 test


  1. […] adverse effects have been experienced with Bravecto Chew for Dogs in MDR-1 gene-deficient collies at 3x the recommended […]