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The essentials

  • Certain breeds are more at risk — Most purebred collies and Australian shepherds have the MDR1 genetic mutation.
  • WSU offers a cheek swab test — Testing is inexpensive and relatively simple, costing $70 for a testing kit that you can mail into the lab at Washington State University.
  • Your vet needs to know— Once you receive your dog’s test results, share them with your veterinarian so they can prescribe medicine in safe doses or choose medications less likely to cause a drug reaction.

Known as Multidrug Resistant 1 (MDR1) or the ABCB1 gene, this genetic mutation causes potentially fatal reactions to common medicines and heartworm preventatives. Dogs with MDR1 most notably experience adverse drug reactions to Ivermectin and may not respond to chemotherapy. Most dogs don’t have this genetic mutation, but certain purebred dogs are at a higher risk, including German shepherds. Learn how to recognize the symptoms and see if testing might be worth it for your dog.

Testing for the gene 

If you have a dog whose breed is at high risk, it’s usually a good idea to test for the MDR1 gene. Washington State University offers a test kit online for $70. It’s a cheek swab test that you mail back to the laboratory and then you can check the results through their online portal. It’s important to share the test results with your vet so they’re best informed to decide which dosages and medications to give your pet and which medicines to avoid.

How dogs acquire the MDR1 gene

While the MDR1 genetic mutation causes adverse drug reactions, it’s not inherently dangerous as long as your vet knows about it. Thus, it’s still considered ethical to breed dogs with the MDR1 genetic mutation. However, if you learn that your purebred dog has the gene, it’s still a good idea to tell the breeder.

What dog breeds have the MDR1 gene?

The MDR1 genetic mutation is more common in certain purebreds. If you have a mixed-breed dog or rescue, you should test them just in case. Dog breeds with a high risk include the following:

  • Collie (70%)
  • Australian shepherd (50%)
  • Long-haired whippet (50%)
  • Miniature Australian shepherd (50%)
  • Silken windhound (30%)
  • McNab (30%)
  • Chinook (25%)
  • Shetland sheepdog (15%)
  • English shepherd (15%)
  • German shepherd (10%)

Drugs to avoid with the MDR1 gene

Once your vet learns that your dog has the MDR1 gene, they will limit or avoid prescribing certain medications.


Ivermectin is an active ingredient in some heartworm prevention, such as Heartguard. If your dog has the MDR1 gene, your vet might prescribe another heartworm prevention that uses a different drug instead. Some dogs with the MDR1 gene can still handle the low doses of ivermectin typically found in heartworm prevention, but always ask your vet first.

Heartworm prevention benefits certainly outweigh the risks: treatment for heartworms is costly and there’s no guarantee it’ll work. Your vet will be the best judge as to whether your dog should take a lower dose of heartworm prevention or a different type, but they’ll still recommend some type of prevention regardless.

Loperamide (Imodium)

Never give this anti-diarrheal medication to your pet if they have the MDR1 mutation. This over-the-counter drug is extremely toxic to affected dogs and can cause fatal drug reactions.

Certain sedatives

Butorphanol and acepromazine may be used as a cough suppressant, pain reliever, and mild sedative in dogs. Acepromazine in particular is often used in conjunction with anesthesia. However, both of these sedatives are extremely dangerous for dogs with the MDR1 gene.


This drug is used to treat congestive heart failure and heart arrhythmia, which is when the heart beats abnormally fast. Digoxin may be toxic to dogs with the MDR1 gene.


Unfortunately, the MDR1 gene mutation aids cancer cells and interferes with chemotherapy. This is because chemotherapy agents, vincristine and doxorubicin, produce enzymatic reactions with the P-glycoprotein affected with MDR1.

The mutation interferes with a specific protein called P-glycoprotein. This protein is responsible for blocking drugs from entering the brain after they are being given. When this protein is not functioning correctly, then this can cause these chemotherapy agents to concentrate in the brain and other organs leading to toxicity.

Dr. Dwight Alleyne

Symptoms to watch for with the MDR1 gene

When the ABCB1 gene operates properly, it filters out toxins from vital organs such as the brain. However, in dogs with the MDR1 gene, these toxins are left unchecked, which causes the adverse drug reactions. Dogs with MDR1 mutations may display these signs after taking a dangerous drug:

  • Vomiting
  • Diarrhea
  • Seizures
  • Tremors
  • Blindness
  • Uncoordinated movement
  • Collapse
  • Death

⚠️ These symptoms can indicate another drug reaction unrelated to the MDR1 gene. Always call your vet immediately if you notice these signs. 

Treatment options concerning the MDR1 gene

Since MDR1 is a genetic abnormality, there isn’t a cure. However, it isn’t life-threatening as long as your vet knows about it. All of the drugs that cause toxicity effects have a safe alternative. In certain circumstances, your vet might even still administer the same drugs, just in a lower dose.

If your dog experiences any type of drug reaction, always call your vet. Reactions can take a turn quickly and may be potentially fatal, so don’t wait for their symptoms to progress before seeking help.

Drug reactions can be scary, but they are easily avoided as long as you know your dog has the MDR1 gene mutation. Testing is recommended if you have a dog breed that’s at a high risk, such as a Collie. Even if your dog does have this genetic mutation, the condition doesn’t affect their daily life. Dogs with MDR1 still live happy, healthy lives — just with carefully formulated prescriptions.

Frequently asked questions

How do I know if my dog has the MDR1 gene?

If you have a dog who is at high risk due to their breed, testing is recommended to avoid drug reactions. Otherwise, you might only know your dog has the gene if they show symptoms. Some dogs may have drug reactions without the MDR1 gene, however, so testing is still recommended so you know for certain.

What breeds have the MDR1 gene?

While dogs of any breed may inherit the MDR1 gene, chances are slim for the dog population as whole — except for certain breeds. Collies carry the highest percentages, with an estimated 70% of this breed inheriting the MDR1 gene. Interestingly, an estimated 1% of the cat population also has the MDR1 gene.

Do vets test for MDR1?

Some vets may provide a blood test for the MDR1 gene. If you decide to test your dog yourself, the cheek swab test kit from WSU is a good idea. Just be sure to share the test results with your vet afterward.

Where is the MDR1 gene located?

ABCB1, the gene that potentially carries the MDR1 genetic mutation, is located on chromosome 14 of a dog’s genome. Every dog passes along two copies of this gene to their offspring, meaning their puppy inherits four. A puppy with even one out of four affected genes may experience some drug resistance, but it won’t be as severe as a dog with all four mutated genes.

How much does a MDR1 test cost?

Washington State University offers a MDR1 test for $70. There may be some other cheaper tests online, but they aren’t necessarily meant to provide medical advice.