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Multidrug Sensitivity (MDR1)

Multidrug Sensitivity (MDR1)

Multidrug Resistance 1, Ivermectin Sensitivity, MDR1 gene defect

The MDR1 gene, or multi-drug resistance gene, codes for a protein that is responsible for transporting potentially harmful chemicals away from the brain. In certain breeds, a mutation occurs in the MDR1 gene that causes sensitivity to Ivermectin, Loperamide, and a number of other drugs. Dogs with this mutation have a defect in the P-glycoprotein that is normally responsible for transporting certain drugs out of the brain. The defective protein inhibits the dog’s ability to remove certain drugs from the brain, leading to a buildup of these toxins. As a result of this toxin accumulation, dogs can show neurological symptoms such as seizures, ataxia, or even death. Dogs that are homozygous for the MDR1 gene, meaning that they have two copies of the mutation, will display a sensitivity to Ivermectin, and other similar drugs. Dogs that are heterozygous, meaning they have only one copy of the mutation, can still react to these drugs at higher doses. The following is a partial list of drugs known to have a negative effect on dogs with the MDR1 mutation:

Acepromazine, Aldosterone, Amitriptyline, Antiemetics, Apomorphine, Buprenorphine, Butorphanol, Chinidin, Cimeditine, Cortisol, Cysolporin A, Dexamethasone, Digoxin, Diltiazem, Domperidone, Doxorubicin, Doxycycline, Ebastine, Erythromycin, Estradiol, Etoposide, Fentanyl, Fexofenadine, Grepafloxacin, Hydrocortisone, Ivermectin, Itraconazole, Ketoconazole, Loperamide, Losartan, Methylprednisolone, Metoclopramide, Metronidazole, Milbemycin, Mitoxantrone, Morhpine, Moxidectin, Ondansetron, Paclitaxel, Phenothiazines, Phenytoin, Quinidine, Ranitidine, Rifampin, Rifamycin, Selamectin, Sparfloxacin, Tacrolimus, Tetracycline, Verapamil, Vinblastine, Vincristine.

Additional drugs beyond this list could have a negative effect on dogs with the MDR1 mutation and it is recommended to consult a licensed veterinarian for more information.

Reading Your Results


These dogs have two copies of the normal gene and will neither develop Multidrug Sensitivity nor pass this mutation to their offspring.


These dogs have one copy of the normal gene and one copy of the mutation associated with this disease. They can react to certain drugs at higher doses and pass the mutation to 50% of its offspring, on average.


These dogs have two copies of the mutation associated with this disease which results in sensitivity to a number of drugs including but not limited to Ivermectin, Loperamide, Doxorubicin, Cyclosporin, Digoxin, Acepromazine, Butorphanol and others.

Additional Details


Autosomal Incomplete Dominance

Affected gene



Ch. 14


G.13726596_13726599del (4bp Del) (CanFam 3.1); C.228_231del AGAT


Barbet JL, Snook T, Gay JM, Mealey KL. ABCB1-1 Delta (MDR1-1 Delta) genotype is associated with adverse reactions in dogs treated with milbemycin oxime for generalized demodicosis. Vet Dermatol. 2009 Apr;20(2):111-4. [PubMed: 19171022]