MDR1: Multi-Drug Resistance Gene


Short Version:

MDR1 is a gene that causes a dangerous sensitivity to some commonly used medications such as Ivermectin. Ivermectin is found in commonly used medications such as Heartgard. About 50% of Australian Shepherds are affected by MDR1.


Longer more detailed version:

MDR1 is a gene responsible for the production of a protein, P-glycoprotein (P-GP). P-GP is a drug efflux pump that aids in controlling drug absorption and distribution specifically in the brain. This causes increased levels of the medications inside the cells.


Imagine a partially clogged drain; when you run water into the sink, the water will build up in the sink similar to the way the medications do in a cells of an MDR1 affected animal.

Sensitivity to (not limited to):

  • Acepromazine - prescription tranquilizer 
  • Butorphanol - pain control 
  • Cyclosporin - immunosuppressive agent 
  • Digoxin - used to treat congestive heart failure 
  • Doxorubicin – Cancer treatment 
  • Doramectin - anti-parasitic medication 
  • Emodepside - anti-parasitic medication 
  • Erythromycin - antibiotic used to treat diarrhea, skin infections and prostate infections 
  • Ivermectin - Found in many anti-parasitic medications such as wormers. 
  • Loperamide – Found in many anti-diarrhea medications such as Imodium. 
  • Milbemycin - used for treatment and prevention of heartworm 
  • Moxidectin - anti-parasitic medication 
  • Paclitaxel - chemotherapy drug 
  • Rifampin - used to treat many bacterial infections  
  • Selamectin - anti-parasitic medication 
  • Vinblastine – Cancer treatment 
  • Vincristine – Cancer treatment

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