MDR1 in Dogs: Which Drugs Are Dangerous, and Which Ones Are Not

By Sarah Bennett · Last updated September 7, 2026 · 12 min read

Information, not medical advice. I am not a veterinarian. The drug list below is read off Washington State University's published tables, linked at every claim, and the point of this page is to help you ask a better question at your next appointment rather than to change anything on your own.

MDR1 in dogs is a genetic quirk that decides how some medicines behave, and it is the one piece of a dog's genetics I think is worth knowing before an older dog is sedated, given chemotherapy, or handed something out of your own medicine cabinet.

Most of what is written about it is stuck twenty years in the past. The story everyone remembers is heartworm medicine killing collies. That story has been overtaken by the actual regulatory record, and the drugs that should worry you now are different, more ordinary, and far more likely to come up in an old dog's year.

The short version: MDR1 is a mutation that lets certain drugs build up in the brain instead of being pumped out. It is common in collies and several herding breeds and turns up in mixed breeds too. The modern heartworm and flea preventives have been tested and cleared for MDR1 dogs at label doses, so that famous danger is mostly retired. What has replaced it is duller and closer to home: loperamide, the active ingredient in Imodium, should be avoided entirely, and a long list of drugs an older dog actually receives, including sedatives, anti-nausea medication, an arthritis drug, and chemotherapy, need dose decisions made by someone who knows the dog's MDR1 status. The only way to know that status is a test.

What the MDR1 mutation actually does

The MDR1 gene, also written ABCB1, builds a pump. That pump sits in the barrier between the bloodstream and the brain, and its job is to push certain drugs back out before they accumulate where they should not be.

A dog carrying the mutation has less of that protection, and depending on what she inherited, possibly none of it. The drug goes in and stays in, and a dose that is routine for other dogs becomes a neurological event. Washington State University, whose lab discovered the mutation, states the consequence plainly: dogs with it "may have severe and life-threatening reactions to some common drugs."

Their diagnostic laboratory puts it less gently, writing that dogs with the mutation "can become very ill or die if given certain medications."

This is not a disease. An MDR1 dog is not sick and never becomes sick from the mutation itself. It only matters at the moment a drug is chosen, which is exactly why it is so easy to reach a dog's twelfth year without anyone having mentioned it.

Which dogs carry it

The breed picture is lopsided. WSU's figure for collies is the one most people have never heard: "Approximately three of every four collies in the United States have the mutant MDR1 gene." That is not an American quirk either, since "The frequency is about the same in France and Australia, so it is likely that most collies worldwide have the mutation."

Beyond collies, WSU's list runs: "Shetland sheepdogs (Shelties), Australian shepherds, old English sheepdogs, English shepherds, German shepherds, long-haired whippets, silken windhounds, and a variety of mixed-breed dogs."

Two things worth noticing in that list.

German shepherds are on it. People file MDR1 under "collie problem" and stop reading, and the German shepherd is one of the most common large breeds in American homes.

Mixed breeds are on it. WSU addresses the cross question directly, saying the mutation "is common in collies and other herding breeds, and while it is less likely to occur in a mixed breed, it is still possible, and testing is recommended to determine the risk to your pet." A shelter dog with some herding somewhere behind her is not covered by "she is not a collie."

And there is no way to reason your way to the answer. WSU is unambiguous: "The only way to know if an individual dog has the mutant MDR1 gene is to have the dog tested."

The one drug to avoid entirely is in your medicine cabinet

Of everything on WSU's table, one entry carries an absolute instruction rather than a dosing conversation, and it is the drug an owner is most likely to give without asking anyone.

Loperamide is the active ingredient in Imodium. WSU's entry for it reads: "At doses used to treat diarrhea, this drug will cause neurological toxicity in dogs with the MDR1 mutation. This drug should be avoided in all dogs with the MDR1 mutation."

Read the shape of that. Not "use with caution," not "reduce the dose," but avoided, in all dogs with the mutation. And the trigger is not some exotic veterinary procedure. It is a dog with loose stool and an owner who has the same box of tablets in the bathroom that they take themselves.

If you take one thing from this page, take that. Human antidiarrheals are not a home remedy for any dog without a vet's say-so, and in an MDR1 dog they are a trip to the emergency room.

The heartworm story is mostly out of date

The reason most people have heard of MDR1 at all is ivermectin and collies, and it is worth being precise about what is still true.

High doses of ivermectin remain genuinely dangerous in MDR1 dogs. What has changed is that the modern preventives are not high doses, and manufacturers have tested them specifically in dogs with the mutation. WSU's table records the regulatory outcome for Heartgard: "The FDA determined Heartgard is safe for dogs with the MDR1 mutation when used at label doses." The same conclusion is recorded for milbemycin products such as Interceptor, for selamectin as Revolution, and for the moxidectin products, where WSU writes that "The FDA has determined that these products are safe for dogs with the MDR1 mutation when used at label doses."

Several common flea and tick products carry the same clearance. WSU records that the makers of Nexgard, Bravecto, Credelio and Simparica each tested in MDR1 dogs, and for Simparica the entry states that "Dogs with the MDR1 mutation are not at increased risk for adverse effects."

The important qualifier in every one of those lines is at label doses, and the exception WSU keeps flagging is other formulations of the same drug, including the large-animal ivermectin sold for livestock. That is where the old horror stories come from, and it is still a real way to hurt a dog.

So the practical translation is this. Keep your dog on her heartworm prevention. The thing to be careful about is the drug given for something else.

The drugs an older dog actually gets

Run down WSU's table and mark the entries that say to contact their pharmacology service for dosing, and what emerges is close to a list of what a thirteen-year-old dog is prescribed in a normal year.

Drug	Where an old dog meets it	WSU's entry
Acepromazine	Sedation, travel, grooming	Contact for dosing
Butorphanol (Torbugesic)	Pain relief and sedation	Contact for dosing
Maropitant (Cerenia)	Nausea, motion sickness, post-op	Contact for dosing
Ondansetron	Nausea, often with chemotherapy	Contact for dosing
Grapiprant (Galliprant)	Arthritis, an FDA-approved NSAID	Contact for dosing
Chemotherapy: vincristine, vinblastine, doxorubicin, vinorelbine	Lymphoma and other cancers	Contact for dosing
Cyclosporine (Atopica)	Skin and immune conditions	Contact for dosing
Apomorphine	Inducing vomiting after a toxin	Contact for dosing

Look at what that covers. Sedation before a dental, which is the most common procedure an older dog has. Anti-nausea medication, which is handed out for vomiting and after almost every anesthetic. An arthritis NSAID. And the entire vinca and doxorubicin group, which is what a dog diagnosed with lymphoma is likely to receive.

None of those entries say the drug is forbidden. They say the dose is a decision that should be made by someone who knows the dog's status, and WSU runs a service for exactly that question. What they cannot do is answer it for a dog whose status nobody has ever checked.

That is the argument for testing an old dog rather than shrugging because she has made it this far without a problem. She has made it this far without needing chemotherapy either.

MDR1 test for dogs: how to get one

The test is a genetic test, so it is done once and the answer never changes. There are two realistic routes.

Direct from the lab that found it. Washington State University's veterinary pharmacology laboratory offers MDR1 testing, and it is the same group whose "Katrina Mealey, who discovered the MDR1 genetic mutation in dogs and cats" still consults with veterinarians on dosing for affected dogs. Going straight to them gets you the single answer you want.

As part of a breed panel. The consumer DNA kits that identify breed mix generally screen MDR1 among their health markers. If you have a mixed-breed senior and have been idly curious about her ancestry anyway, this is the one result on those panels with an immediate, practical use. For a dog already twelve, most of the rest of a breed-risk report is telling you what to watch for during years that have already happened. MDR1 is the exception, because it applies to the next prescription rather than to the next decade.

Whichever route, the step that matters comes after the result arrives: get it into her chart, and onto the record you bring to appointments, so it sits in front of the person writing the prescription rather than in an email from four years ago.

What a positive result actually changes

Less than people fear, and more than they expect.

It does not mean your dog is ill, cannot be sedated, or cannot have chemotherapy. Affected dogs are anesthetized and treated for cancer routinely. It means the drugs and doses are chosen with the mutation in mind, and that some substitutions get made quietly by a vet who has the information.

Day to day it means one hard rule and one habit. The hard rule is loperamide, never. The habit is saying "she is MDR1 affected" out loud at the start of any appointment where a new drug might be prescribed, including at an emergency clinic that has never seen her, where nobody will have read her file.

Frequently Asked Questions

What is the MDR1 gene in dogs?

MDR1, also called ABCB1, codes for a pump that keeps certain drugs out of the brain. Dogs with a mutated version lack that protection, so some medications reach concentrations in the brain that cause neurological toxicity at doses other dogs tolerate. WSU describes affected dogs as at risk of severe and life-threatening reactions to some common drugs.

Which breeds have the MDR1 mutation?

WSU reports that roughly three of every four collies in the United States carry it, with similar frequencies elsewhere, and lists Shetland sheepdogs, Australian shepherds, old English sheepdogs, English shepherds, German shepherds, long-haired whippets, silken windhounds, and mixed-breed dogs as also affected. Testing is the only way to know about an individual dog.

Is Imodium safe for dogs?

Not for a dog with the MDR1 mutation, and not as a home remedy for any dog without veterinary direction. WSU states that loperamide at the doses used to treat diarrhea will cause neurological toxicity in dogs with the mutation and should be avoided in all of them.

Is heartworm medication safe for a collie?

At label doses, the FDA has determined that Heartgard, Interceptor and Revolution are safe in dogs with the MDR1 mutation, and the same conclusion applies to the moxidectin products. The danger sits with other formulations and higher doses, particularly livestock ivermectin, not with the monthly preventive your vet dispenses.

Should I test a senior dog for MDR1?

There is a case for it if she is a herding breed or mix, and a stronger one if anything is coming up that involves sedation, chemotherapy, or new medication. The result never changes and it never expires, so a test at eleven is as useful as a test at one, and it is most useful in the year she needs the most drugs.

Does the MDR1 mutation make a dog sick?

No. It causes no illness on its own and no symptoms. It only matters at the moment a drug is chosen, which is why so many affected dogs reach old age with nobody having mentioned it.

Sources

Washington State University, Program in Individualized Medicine. Dog breeds commonly affected by MDR1 mutation. Breed frequencies, the mixed-breed question, and the statement that testing is the only way to know. prime.vetmed.wsu.edu
Washington Animal Disease Diagnostic Laboratory, WSU. Problem medications for dogs. The drug-by-drug table quoted above, including the loperamide entry and the FDA determinations on heartworm and flea preventives. waddl.vetmed.wsu.edu
Washington Animal Disease Diagnostic Laboratory, WSU. MDR1 in Dogs. Overview and testing. waddl.vetmed.wsu.edu

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About the authors

Senior Dog Daily is written by Sarah Bennett and Leo Bennett, with their Border Collie Lucy as the reason it started. Neither of us is a veterinarian. This page exists because the version of the MDR1 story most of us absorbed, the one about heartworm pills and collies, is the part that has since been resolved, while the parts that matter for an old dog, a sedative before a dental and a box of Imodium in the bathroom, are the ones nobody repeats.