It’s Not Just Collies
When people hear the name of this site - Collie Drug Reactions - they sometimes make an understandable assumption. They think this is a Collie problem. A Collie-specific thing. Something that doesn’t apply to their Australian Shepherd, their Sheltie, their Border Collie mix from the shelter, their “Lab mix” with the suspiciously pointy ears and intense stare.
They’re wrong. And that assumption has killed dogs.
The MDR1 mutation doesn’t care what you call the breed. It doesn’t care whether your dog has AKC papers or was found wandering a parking lot. It follows genetics, not labels. And the genetics are far more widespread than most people - including most veterinarians - realize.
I named this community after Collies because that’s where my story started. Cooper was a Collie. But the families who’ve found us include owners of Australian Shepherds, Shelties, Border Collies, German Shepherds, Old English Sheepdogs, and dogs whose breed is listed as “your guess is as good as mine.” The mutation doesn’t discriminate. Neither should our awareness.
The High-Risk Breeds
These breeds carry the MDR1 mutation at rates high enough that any dog of these breeds should be considered at risk until proven otherwise by testing. Our MDR1 testing guide walks you through how to get tested, what the results mean, and what to do next.
Rough and Smooth Collies (50-75% affected)
This is where the research started and where the highest prevalence exists. Up to three-quarters of Collies carry at least one copy of the mutant gene. If you have a Collie and haven’t tested for MDR1, you’re gambling with odds that would make a casino blush.
Cooper was a Rough Collie. He was never tested. I didn’t know the test existed. I think about that every single day.
Australian Shepherds (25-50% affected)
Aussies are the second most commonly affected breed, and because they’re one of the most popular breeds in America, they may actually account for more MDR1-related deaths than Collies in raw numbers. Luna, the blue merle Aussie from our community stories, died from over-the-counter Imodium - a drug her owner didn’t even get from a vet.
Shetland Sheepdogs (5-15% affected)
The prevalence in Shelties is lower than in Collies or Aussies, but it’s high enough that every Sheltie should be tested. Finn, the Sheltie whose story appears in our community testimonials, died from a routine dewormer at his annual checkup. His family asked the vet if it was safe. She didn’t know to check.
Old English Sheepdogs (5-15% affected)
Less commonly discussed in the MDR1 conversation, but absolutely at risk. The mutation follows the same pattern - the same broken P-glycoprotein pump, the same inability to clear certain drugs from the brain.
English Shepherds (15-20% affected)
A working breed that often flies under the radar in health discussions. Whiskey, the English Shepherd from Vermont whose story appears on our site, was described by his owner as “the smartest dog I’ve ever known.” His intelligence couldn’t save him from a drug his body couldn’t process.
Longhaired Whippets (50-65% affected)
This one surprises people. Whippets aren’t herding dogs, so why do they carry a herding breed mutation? The answer lies in the breed’s development history. The Longhaired Whippet’s founding stock likely included herding breed crosses, and the MDR1 mutation came along for the ride.
Silken Windhounds (30-50% affected)
Closely related to the Longhaired Whippet, and carrying the mutation for the same historical reasons. Another sighthound breed that most people wouldn’t associate with MDR1 risk.
The Moderate-Risk Breeds
These breeds carry the mutation at lower but still significant rates. Testing is strongly recommended.
Border Collies (1-5% affected)
The prevalence is lower than many people expect given the Border Collie’s status as the archetypal herding breed. But “low prevalence” is cold comfort if your Border Collie happens to be in that percentage. Gus, from our community stories, was a Border Collie who spent his last year dealing with liver damage from a drug reaction that was probably MDR1-related.
German Shepherds (5-15% affected)
This is a big one. German Shepherds are the second or third most popular breed in America depending on the year. A 5-15% prevalence rate across millions of dogs means a staggering number of affected individuals. And yet, when was the last time you heard a German Shepherd breeder talk about MDR1 testing?
McNab Shepherds (prevalence not well studied)
A lesser-known herding breed from Northern California. Limited data exists, but the breed’s herding background puts it firmly in the at-risk category.
Mixed Herding Breeds (unknown but potentially high)
This might be the most important category on this list. I’ll address it separately below.
The Mixed-Breed Problem
Here’s where things get complicated - and where the most dogs slip through the cracks.
According to the ASPCA, approximately 3.1 million dogs enter shelters every year in the United States. A significant percentage of those dogs are mixed breeds with unknown parentage. Many of them have herding breed in their background. Some of them look like Labs. Some look like Pit Bull mixes. Some look like nothing in particular.
You cannot tell by looking at a dog whether they carry the MDR1 mutation.
Let me say that again, because it’s critical: you cannot tell by looking at a dog whether they carry the MDR1 mutation. The only way to know is to test.
I’ve seen dogs at my rescue who looked nothing like Collies test positive for the mutation. I’ve seen dogs who looked like textbook Collies test negative. Appearance is not a genetic test. Breed labels on shelter intake forms are not genetic tests. Your best guess based on ear shape and coat color is not a genetic test.
If your dog has any chance of herding breed ancestry - and unless you have papers going back generations, you don’t know for sure - get them tested. The test costs less than a vet visit. The results last a lifetime.
Why Does the Mutation Persist?
People sometimes ask me: if this mutation is so dangerous, why hasn’t it been bred out? Why do so many dogs still carry it?
The answer involves some basic genetics. The MDR1 mutation is what geneticists call a “founder effect.” It originated in a single dog, probably in the 1800s, who happened to be an ancestor of many modern herding breeds. As these breeds were developed from a relatively small number of founding dogs, the mutation spread through the population.
The mutation persists because it doesn’t cause any problems on its own. An MDR1-affected dog who never receives a P-glycoprotein substrate drug lives a perfectly normal, healthy life. The mutation is only dangerous when it interacts with specific medications. For centuries, before these medications existed, the mutation was invisible and harmless.
For more on the genetics behind this mutation and how it’s inherited, The Herding Gene provides detailed, research-based information about herding breed genetics that’s written accessibly for dog owners and breeders alike.
This is also why simply trying to eliminate the mutation through breeding isn’t straightforward. In breeds where 50-75% of dogs carry at least one copy, removing all carriers from the breeding pool would devastate the gene pool and create a host of other genetic problems. The better approach is universal testing and informed medication choices.
What This Means for Veterinarians
If you’re a vet reading this - and some of our most dedicated community members are veterinary professionals - the breed list above should change how you approach prescribing.
The reflexive assumption should not be “this is probably fine.” For any dog that could be a herding breed or herding breed mix, the default should be caution. Ask about MDR1 status. If they don’t know, recommend testing. If testing isn’t possible before treatment is needed, choose safer alternatives whenever they exist.
Our article on what veterinarians may not know about drug sensitivity goes deeper into the education gap and how to bridge it without damaging the vet-client relationship.
The Shelter Dog Problem
Dogs adopted from shelters almost never come with MDR1 test results. If you work in shelter medicine or rescue, consider adding MDR1 testing to your intake protocol for any dog that could have herding breed ancestry. The cost per test is minimal compared to the cost of a drug reaction - both in veterinary bills and in the trust families place in your care.
What This Means for You
If you’re reading this and you have a dog from any of the breeds listed above - or a mixed breed with any herding ancestry - here’s your action plan:
- Get tested. Order a kit today. Our testing guide explains every option.
- Tell your vet. Before the results even come back, let your vet know you’re testing and that your dog may be at risk.
- Prepare for vet visits. Our vet visit checklist gives you the questions to ask and the information to bring to every appointment.
- Know the emergency plan. If the worst happens, our emergency guide walks you through what to do in the first 60 minutes.
- Spread the word. Tell every dog owner you know. Especially the ones who say “Oh, my dog isn’t a Collie, so this doesn’t apply to me.” It might apply to them more than they think.
Cooper’s Legacy, Beyond Collies
I started this community because of a Collie. I named it after Collies. But every time I hear from a family who lost a German Shepherd or an Aussie mix or a shelter dog with an unknown background, I’m reminded that this is bigger than one breed.
The MDR1 mutation doesn’t read breed standards. It doesn’t check pedigrees. It hides in DNA, waiting quietly, doing no harm at all - until the day a well-meaning vet prescribes the wrong drug and a family loses their best friend.
Test your dog. Whatever breed they are. Whatever breed you think they are. Test them.
Because the label on the shelter paperwork doesn’t matter. The breed the groomer guessed doesn’t matter. What matters is what’s written in your dog’s genes - and the only way to read it is to test.
Have a breed not listed here that you believe carries the MDR1 mutation? We want to hear about it. Contact us with your experience, and help us keep this information as complete as possible.