MDR1 Gene Mutation in Dogs: Why Some Breeds React to Drugs

Most dogs handle common veterinary medicines without trouble, but some dogs can have serious, even life-threatening, reactions to drugs that are usually considered safe. Sometimes the reason is a change in a single gene called MDR1. This mutation is especially common in Collies, Australian Shepherds, Shelties, and related herding breeds, yet it also turns up in mixed-breed dogs who look nothing like a Collie. A simple DNA test can tell you whether your dog carries it. This guide explains what the MDR1 gene does, why the mutation matters, which breeds are affected, how testing works, which drugs call for caution, and when to seek emergency help.

Important: This article shares general educational information for pet owners and is not a substitute for an in-person veterinary examination, diagnosis, or treatment. For any medical concern or emergency, contact your veterinarian promptly.

What the MDR1 Gene Does

MDR1 stands for multidrug resistance 1. Scientists now usually call the gene ABCB1, but you will see both names used for the same gene. It carries the instructions for building a protein called P-glycoprotein, often shortened to P-gp.

P-glycoprotein works like a molecular pump. It sits in cell membranes in several parts of the body and pushes certain drugs back out of those cells, shaping how much of a medicine reaches different tissues.

Where P-glycoprotein works

• The blood-brain barrier: the specialized lining of the blood vessels in the brain, where P-gp pumps many drugs back into the bloodstream so they do not build up in brain tissue.
• The intestines: where it limits how much of some drugs is absorbed from the gut.
• The liver and kidneys: where it helps move certain drugs into bile and urine so they can leave the body.

Of all these roles, the blood-brain barrier is the one that matters most for drug reactions.

What Happens When the Gene Is Mutated

The MDR1 mutation is a small deletion in the gene's code. Because a few letters of the instructions are missing, the cell cannot build a working P-glycoprotein pump.

Without that pump, some drugs that would normally be pushed back out of the brain are able to accumulate there. Higher levels of these drugs in brain tissue can affect the nervous system, leading to signs such as drooling, wobbliness, tremors, disorientation, and in severe cases seizures or coma.

This is why owners and veterinarians sometimes call the condition multidrug sensitivity. The mutation itself does not make a dog sick. Affected dogs are usually healthy, and problems only appear after exposure to certain medicines or products.

Breeds Commonly Affected

The mutation is believed to trace back to a single ancestral dog, so it is found most often in herding breeds and breeds that share their ancestry. Breeds in which the MDR1 mutation is well documented include:

• Collies, both rough and smooth
• Australian Shepherds, including Miniature Australian Shepherds
• Shelties
• Long-haired Whippets and Silken Windhounds
• English Shepherds and McNabs
• German Shepherds and White Swiss Shepherds
• Border Collies, in which the mutation is found less often but still occurs

This is not a complete list, and how common the mutation is varies widely from breed to breed. Within any breed, some dogs have it and many do not, so breed alone cannot tell you a particular dog's status.

Mixed-breed dogs can carry it too

A mixed-breed dog with herding breed ancestry, even several generations back, can carry the mutation. Because appearance is a poor guide to ancestry, many veterinarians suggest testing any mix that might have herding breed heritage.

How MDR1 Is Inherited

Every dog has two copies of the MDR1 gene, one inherited from each parent. Each copy can be normal or carry the mutation. That gives three possible results:

• Normal/normal: both copies are normal. The dog has typical P-glycoprotein function and is not expected to have MDR1-related drug sensitivity, although any dog can still react to a medicine for other reasons.
• Normal/mutant (one copy): the dog is often described as a carrier. These dogs may still be more sensitive than normal to certain drugs, particularly at higher amounts, so veterinarians generally plan their medicines with caution.
• Mutant/mutant (two copies): the dog has little or no working P-glycoprotein at the blood-brain barrier and is the most likely to have serious reactions to drugs of concern.

A dog with two mutant copies will always pass one mutant copy to every puppy. A dog with one copy can pass on either the normal or the mutant version. Responsible breeders often test breeding dogs, but each puppy benefits from having its own status confirmed.

Testing Your Dog: Cheek Swab or Blood Sample

MDR1 status is determined with a DNA test. Because it looks at the genes themselves, the result does not change with age, and a dog only needs to be tested once in its lifetime. For an overview of how canine genetic testing works more broadly, see our guide to DNA and genetic testing for dogs and cats.

1. Talk with your veterinarian: they can recommend a reputable laboratory and explain how the result will affect your dog's care.
2. Collect the sample: most tests use a cheek swab, which is quick and painless, while some laboratories use a small blood sample drawn at the clinic.
3. Send it to the laboratory: follow the laboratory's instructions closely, since contamination with food or another dog's saliva can affect the sample.
4. Review the result: your report will show normal/normal, normal/mutant, or mutant/mutant.
5. Record it everywhere: add the result to your dog's medical records and keep a copy you can share easily.

Drug Classes That Call for Caution

Many drugs are moved by P-glycoprotein, but only some are a real concern for dogs with the mutation. Your veterinarian will decide which medicines are appropriate. Every medicine should be given only as prescribed by your veterinarian. Classes commonly discussed include:

• Certain macrocyclic lactone antiparasitics: this family includes ingredients in many heartworm preventives. Heartworm products labeled for dogs are generally considered safe for dogs with the mutation when used exactly as directed, but the higher amounts sometimes used for skin mite conditions are a well-known risk.
• Some chemotherapy agents: several drugs used in cancer care rely on P-glycoprotein for clearance, so oncologists typically check MDR1 status before starting therapy.
• Certain sedatives and pain medicines: some drugs commonly used for sedation and pain control may have stronger or longer-lasting effects. See our guide to anesthesia safety for pets.
• Certain antidiarrheals: some over-the-counter human antidiarrheal products contain an opioid-type ingredient that can enter the brain in affected dogs and cause neurologic signs. Never give any human antidiarrheal without your veterinarian's direction.

Because these lists are updated as research continues, rely on your veterinarian rather than an online list when deciding what is safe.

A Hidden Risk: Dewormer Products Not Made for Dogs

One of the most common ways dogs with the MDR1 mutation are poisoned is by eating concentrated dewormer products not made for dogs. Some contain macrocyclic lactone ingredients at strengths far beyond what a dog should ever receive. Even a small lick can be dangerous for a sensitive dog.

Dogs may find these products in sheds or garages, or chew on a dropped tube or syringe of dewormer paste. Keep such products sealed and stored out of reach, clean up spills right away, and never give your dog any dewormer that was not prescribed or recommended for dogs by your veterinarian.

If you think your dog has eaten any such product, contact your veterinarian or an emergency clinic immediately, even if your dog seems fine. Our suspected pet poisoning action plan walks through what to do in the first minutes.

Signs of Drug Sensitivity and Neurologic Toxicity

Signs of a reaction can appear within hours of exposure or more gradually, and range from mild to severe. Signs that warrant urgent veterinary attention include:

These signs are not specific to MDR1 and can have many causes. Whatever the reason, they warrant prompt veterinary assessment. Affected dogs often need supportive hospital care while the drug slowly leaves the body.

When to Call Your Veterinarian

Contact your veterinarian or an emergency clinic right away if your dog

• Has eaten any dewormer, parasite product, or medicine not prescribed for it
• Shows drooling, stumbling, tremors, or confusion after starting a new medicine
• Has dilated pupils or seems unable to see
• Has a seizure, collapses, has trouble breathing, or will not wake normally

Bring the product packaging or label with you if you can, and tell the team your dog's MDR1 status or breed background. Do not try to make your dog vomit unless a veterinarian tells you to.

Schedule a routine conversation if

• You have a herding breed or herding mix whose MDR1 status is unknown
• Your dog is about to start a new medicine, procedure, or cancer therapy
• You are planning to use a parasite product and are unsure whether it suits your dog
• Your dog has reacted strongly to a medicine in the past

Common Mistakes and Myths

Myth: Only purebred Collies are affected.

Fact: the mutation occurs across several breeds and in many mixed-breed dogs with herding ancestry.

Myth: A dog with one copy is completely safe.

Fact: dogs with one copy may still be more sensitive than normal to some drugs, so veterinarians plan their medicines with care.

Mistake: Telling only your regular veterinarian.

Emergency clinics, specialists, boarding facilities, and pet sitters should all know your dog's status. Keep a note in your phone too.

Frequently Asked Questions

Can my dog with the MDR1 mutation still get heartworm preventives?

In most cases, yes. Heartworm preventives labeled for dogs are generally considered safe for MDR1 dogs when given exactly as directed. Ask your veterinarian which product fits your dog, and give it only as prescribed by your veterinarian.

Does the mutation shorten a dog's life?

The mutation itself does not cause illness. Dogs with it can live normal lives with careful medicine planning.

If my dog tests normal/normal, do I need to worry about drug reactions?

MDR1-related sensitivity is not expected, but any dog can react to a medicine for other reasons. Always use medicines only as prescribed by your veterinarian and report any new signs promptly.

Key Takeaways

The MDR1, or ABCB1, gene builds P-glycoprotein, a pump that keeps many drugs out of the brain. Dogs with the mutation can develop serious neurologic signs after exposure to certain antiparasitics at higher amounts, some chemotherapy agents, certain sedatives, and some over-the-counter human antidiarrheals. Collies, Australian Shepherds, Shelties, related herding breeds, and herding mixes are most often affected. A one-time cheek swab or blood DNA test shows whether your dog is normal/normal, carries one copy, or has two copies. Share the result with every veterinarian and caregiver, keep concentrated dewormer products not made for dogs securely out of reach, and contact your veterinarian or an emergency clinic right away if you see drooling, stumbling, tremors, blindness, or seizures.