MDR1 is a gene that produces a protein responsible for pumping certain drugs and toxins out of your dog’s brain. When a dog carries a mutation in this gene, that pump doesn’t work properly, allowing medications to build up to dangerous or even fatal levels in the nervous system. Roughly 70% of Collies carry the mutation, along with significant percentages of Australian Shepherds, Shetland Sheepdogs, and several other breeds.
How the MDR1 Gene Protects Your Dog’s Brain
The MDR1 gene provides the blueprint for a protein called P-glycoprotein, which acts as a gatekeeper at the blood-brain barrier. This protein sits in the walls of blood vessels that supply the brain and actively pushes foreign substances, including many common medications, back out into the bloodstream before they can reach sensitive nerve tissue. It works like a bouncer, recognizing a wide range of structurally different chemicals and escorting them out of cells.
Dogs with the MDR1 mutation have a specific error in the gene’s DNA: a deletion that shifts the entire reading frame, preventing the body from producing any functional P-glycoprotein at all. The gene still gets transcribed, but the message is garbled enough that no working protein is made. Without this pump, drugs that would normally be kept out of the brain pass freely into it. In studies comparing normal dogs to those with the mutation, brain concentrations of the antiparasitic drug ivermectin were 80 to 90 times higher in affected animals. In normal dogs, ivermectin levels in the brain are about one-tenth of blood levels. In sensitive Collies, brain concentrations can actually exceed those found in the blood and liver.
Breeds Most Commonly Affected
The mutation is most heavily concentrated in herding breeds and their relatives. Based on testing data from Washington State University, these are the approximate frequencies:
- Collie: 70%
- Australian Shepherd (standard and mini): 50%
- Long-haired Whippet: 50%
- McNab: 30%
- Silken Windhound: 30%
- Chinook: 25%
- Shetland Sheepdog: 15%
- English Shepherd: 15%
- German Shepherd: 10%
- Herding breed crosses: 10%
- Old English Sheepdog: 5%
- Border Collie: less than 5%
- Mixed breed dogs: 5%
The mutation has also been identified in some Golden Retrievers and Siberian Huskies, though not enough dogs have been tested to calculate a reliable frequency. If your dog is a mix that includes any of these breeds, testing is worth considering.
How the Mutation Is Inherited
MDR1 follows a straightforward inheritance pattern. Every dog gets two copies of the gene, one from each parent, and the result falls into three categories:
- Normal/Normal: Two working copies. No drug sensitivity.
- Mutant/Normal (heterozygous): One working copy, one mutated. These dogs have reduced P-glycoprotein function and can still experience toxicity with certain drugs, particularly at higher doses. Dose reductions are recommended for several medications.
- Mutant/Mutant (homozygous): No working copies. These dogs are most severely affected and at highest risk for life-threatening reactions.
This is an important distinction. Carriers with just one copy of the mutation are not “safe.” They still need dose adjustments for drugs like acepromazine, butorphanol, and chemotherapy agents. The difference is a matter of degree, not an all-or-nothing situation.
Medications That Pose a Risk
The most dangerous scenario for an MDR1-affected dog involves drugs that are normally kept out of the brain by P-glycoprotein. When the pump is absent, these drugs accumulate in the nervous system and cause toxicity. The list includes several categories of medication your dog could encounter in routine veterinary care.
One drug that should be avoided entirely is loperamide, sold over the counter as Imodium. At standard doses used for diarrhea, it causes neurological toxicity in dogs with the mutation. This is the most important one for owners to know, since it’s something you might give your dog at home without a vet’s involvement.
Other medications requiring dose adjustments include acepromazine (a common sedative used before surgery), butorphanol (a pain reliever often used alongside sedation), several chemotherapy drugs used in cancer treatment, the anti-nausea medication maropitant, the anti-inflammatory grapiprant, and cyclosporine (used for skin conditions and immune disorders). These drugs aren’t necessarily off-limits, but your vet needs to know your dog’s MDR1 status to prescribe them safely.
Drug combinations add another layer of risk. Certain medications, when used together, can increase the total concentration of individual drugs in the body. Ketoconazole (an antifungal), cyclosporine, and the flea preventive spinosad are examples. These combinations can cause adverse reactions even in dogs without the mutation, but the risk is amplified in MDR1-affected dogs.
One common concern is accidental exposure. If your dog eats horse manure from a recently dewormed horse, for instance, it could ingest ivermectin at doses far higher than what’s found in dog heartworm preventives. This type of unintentional exposure can be life-threatening.
Heartworm Prevention Is Still Safe
All heartworm prevention products labeled for dogs in the United States have been tested in dogs with the MDR1 mutation, as required by the FDA, and are safe at their labeled doses. The antiparasitic drugs used in these products, including ivermectin, selamectin, milbemycin, and moxidectin, cause problems only at much higher doses, such as those used to treat mange.
To put the numbers in perspective: the dose of ivermectin used for heartworm prevention is about 6 micrograms per kilogram of body weight. The doses used for mange treatment range from 300 to 600 micrograms per kilogram, roughly 50 to 100 times higher. It’s that higher range that causes neurological toxicity in affected dogs. So skipping heartworm prevention out of MDR1 concerns would actually put your dog at greater risk. One exception to note: combination products containing spinosad should be used cautiously in dogs that carry even one copy of the mutation, due to potential drug interactions.
What Toxicity Looks Like
When an MDR1-affected dog receives a problem drug at an unsafe dose, the signs are neurological. The brain contains nerve cells that respond to a signaling chemical called GABA, which normally has a calming, inhibitory effect on nerve activity. Drugs like ivermectin work in parasites by overstimulating these same pathways. In a normal dog, P-glycoprotein keeps these drugs out of the brain entirely. In a dog without the pump, the drugs flood the nervous system.
Symptoms typically include excessive drooling, disorientation, stumbling or loss of coordination, tremors, and dilated pupils. In more severe cases, dogs may become unresponsive, experience seizures, go blind temporarily, or fall into a coma. The severity depends on which drug was involved, the dose, and whether the dog is homozygous or heterozygous for the mutation. Without treatment, severe toxicity can be fatal.
Getting Your Dog Tested
MDR1 testing is a simple DNA test that can be done from a cheek swab collected at home or from a blood sample at your vet’s office. Washington State University, which pioneered much of the research on this mutation, offers testing for $70 with pharmacology consulting included. UC Davis offers the test for $55, with discounts for multiple dogs. Results tell you definitively whether your dog is normal/normal, mutant/normal, or mutant/mutant.
If your dog belongs to any of the breeds listed above, or is a mix that could include those breeds, testing before any surgical procedure or new medication is the simplest way to prevent a crisis. Once you have the results, make sure every veterinarian and emergency clinic your dog might visit has the information on file. If your dog tests positive for even one copy of the mutation, a note on their medical record ensures that any vet who sees them in an emergency will know to adjust drug choices and doses accordingly.