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MDR1 Medication Sensitivity

The MDR1 gene mutation causes a defect to a drug pumping protein that plays an important role in limiting drug absorption and distribution (particularly to the brain). Dogs with the MDR1 mutation may have severe adverse reactions to some commonly used medications.

Found in

1 in 50 dogs

in our testing

Key Signs

Some medications may cause prolonged sedation, stupor, coma, seizures.

Age of Onset

At birth

Present at birth


Autosomal Dominant

For autosomal dominant disorders, dogs with one or two copies of the disease variant are at risk of developing the condition. Inheriting two copies of the risk variant may make the risk higher or the condition more severe. They may produce puppies affected with the disorder if bred.

Likelihood of the Condition

High likelihood

At risk dogs are highly likely to show signs of this disease in their lifetime.

What to Do

Here’s how to care for a dog with MDR1

Partner with your veterinarian to make a plan regarding your dog’s well-being, including any insights provided through genetic testing. If your pet is at risk or is showing signs of this disorder, then the first step is to speak with your veterinarian.

For Veterinarians

Here’s what a vet needs to know about MDR1

Dogs that carry this mutation are asymptomatic until they are exposed to a medication that uses the pump that is rendered defective by the mutation in the MDR1 (also called ABCB1) gene. Drugs known to use this P-glycoprotein pump are macrocyclic lactones (antiparasitic drugs), loperamide (antidiarrheal), erythromycin (antibiotic), acepromazine (tranquilizer), butorphanol (opioid), and certain drugs used in cancer treatment (vincristine, vinblastine, and doxorubicin). When these medications are administered, they accumulate in the brain which results in the adverse reactions. Typical symptoms include tremors, loss of balance, seizures, obtundation, excessive salivation, dilated pupils, and bradycardia. If untreated, the condition may lead to respiratory arrest, coma or death. Because dogs with one copy of the mutation will have some P-glycoprotein function, the most severe cases tend to occur in dogs that have two copies of the mutation and therefore lack any functional P-glycoprotein pumps. However, the condition can still be very severe in dogs that have only one copy of the mutation.

For dogs with copies of the MDR1 mutation it is important to be aware of medications which may result in an adverse reaction. These include: Acepromazine, Butorphanol, Doxorubicin, Doramectin, Emodepside, Erythromycin, Ivermectin, Loperamide, Milbemycin, Moxidectin, Paclitaxel, Rifampin, Selamectin, Vinblastine, Vincristine. It is best to explore alternative medications that will not use this P-glycoprotein pump or, at minimum, consider reduction of the dose to levels safe for an MDR1 positive dog.

For Breeders

Planning to breed a dog with this genetic variant?

There are many responsibilities to consider when breeding dogs. Regardless of test results it is important that your dog is in good general health and that you are in a position to care for the puppies if new responsible owners are not found. For first time or novice breeders, advice can be found at most kennel club websites.

This disease is autosomal dominant meaning that one copy of the mutation is needed for signs of this disorder to occur. For some breeds where the MDR1 mutation frequency is particular high, breeders may consider matings using dogs that have one or two copies of the MDR1 mutation to maintain genetic diversity within their breed. IMPORTANT: Resulting puppies must be tested for the MDR1 mutation to ensure safe future medical treatment. If a dog with one copy of the MDR1 mutation is bred with a clear dog with no copies of the MDR1 mutation, about half of the puppies will have one copy and half will have no copies of the MDR1 mutation. If a dog with two copies of the MDR1 mutation is bred with a clear dog, the resulting puppies will all have one copy of the mutation. Please note: It is possible that disease signs similar to the ones caused by the MDR1 mutation could develop due to a different genetic or clinical cause.

Technical Details

Variant Deletion
Chromosome 14
Coordinate Start 13,726,599
Coordinate End 13,726,596

All coordinates reference CanFam3.1

References & Credit

Credit to our scientific colleagues:

Mealey, K. L., Bentjen, S. A., Gay, J. M., & Cantor, G. H. (2001). Ivermectin sensitivity in collies is associated with a deletion mutation of the mdr1 gene. Pharmacogenetics, 11(8), 727–733. View the article