How the MDR1 Mutation Actually Works: P-glycoprotein and the Blood-Brain Barrier
Understand the biology behind every drug warning on this site — what ABCB1 does, how the broken pump lets toxins into the brain, and why dose changes everything.
We Share Our Stories So Others Don't Have to Live Ours
Real stories from families who've lost dogs to drug sensitivities. Community support, vet visit checklists, and the information we wish we'd had.
This Site Was Born From Loss
In March 2019, my beautiful Collie Cooper was given Ivermectin for a skin condition. The vet meant well. I trusted her completely. Neither of us knew that Cooper carried the MDR1 gene mutation that makes certain drugs deadly for Collies and other herding breeds.
Cooper was gone within 48 hours.
I spent months blaming myself. Why didn’t I know? Why didn’t the vet know? Why isn’t this common knowledge when it kills dogs every single day?
We’re Not Experts. We’re Families Who’ve Been Through This.
This isn’t a medical website run by scientists in white coats. This is a community of dog owners who’ve held their best friends as they slipped away, often because of a medication that should never have been prescribed.
We share our stories because each one might save a dog’s life. We share what we learned too late so you can learn it in time.
“I found this community after losing my Sheltie Bailey. Reading other families’ stories - knowing I wasn’t alone in my grief and guilt - that’s what got me through. Now I make sure every vet I see knows about MDR1 before they touch my dogs.” - Jennifer M., Michigan
What You’ll Find Here
Real stories from real families - No sugarcoating. No clinical detachment. Just honest accounts of what happened to our dogs and what we wish we’d known.
Practical tools - The vet visit checklist I wish I’d had. The questions to ask. The drugs to refuse.
Community support - Groups and resources where you can connect with others who understand exactly what you’re going through.
The information your vet might not have - We love our vets, but many don’t know about MDR1. That’s not their fault. It’s a gap in veterinary education we’re trying to help fill.
The Drugs That Took Our Dogs
These are the medications that most commonly cause fatal reactions in MDR1-affected dogs:
- Ivermectin - Often used for mange, parasites, heartworm at high doses
- Loperamide (Imodium) - A common over-the-counter anti-diarrheal that crosses the blood-brain barrier without P-gp protection
- Acepromazine - A widely-used sedative
- Certain chemotherapy drugs - Vincristine, doxorubicin, vinblastine, and others that require dose adjustment in MDR1-positive dogs
There are many more. Before any medication is given to your Collie, Aussie, Sheltie, or other herding breed, ask: “Is this safe for MDR1 dogs?” For most of these dangerous drugs, safer alternatives are available - you just need to know what to ask for.
Get Your Dog Tested
A simple cheek swab can tell you if your dog carries the MDR1 mutation. It costs less than one emergency vet visit. It takes five minutes. It could save your dog’s life. Our MDR1 testing guide covers everything from ordering a kit to understanding your results.
For detailed scientific information about MDR1 testing and the genetics behind this mutation, visit our partner site The Herding Gene for research-based guidance.
Every single one of us wishes we’d done it sooner.
Selected breeder resources
Additional breeder resources from our wider canine network.
- Border Collie working breeder - A practical breed resource for owners who also need to understand MDR1 and medication risk.
Community Stories
Cyclosporine, Atopica and Allergy Medications in MDR1 Dogs
Atopica is a P-glycoprotein substrate that lands on the MDR1 problem list — but the nuance matters. Here's what owners managing a herding breed's chronic allergies need to flag.
Accidental Ivermectin Poisoning in Herding Dogs: Horse Paste, Cattle Dewormer and Manure
The heartworm dose of ivermectin is safe. The real killer is accidental high-dose exposure on farms. Here's how much is dangerous and what to do in the first hour.
Are Flea and Tick Chews Safe for MDR1 Dogs? NexGard, Bravecto and Simparica Explained
Owners panic that isoxazoline chews are an MDR1 risk. They're not — but a separate FDA warning still applies. Here's how to untangle the two and choose a protocol.
Anti-Nausea and Anti-Vomiting Drugs in MDR1 Dogs: Cerenia, Ondansetron and Metoclopramide
Car sickness, post-op nausea, and tummy upsets are routine — but several anti-nausea drugs need dose care in MDR1 dogs. Here is how to handle each one.
Pain Management for MDR1 Dogs: Opioids, NSAIDs, and the Butorphanol Trap
How to control post-surgical and chronic pain in an MDR1 dog safely — which NSAIDs and opioids work, and why butorphanol is the one to question.
The WSU Problem Drug List Decoded: Every Medication MDR1 Owners Should Question
A plain-English guide to the Washington State University MDR1 problem drug list, sorted into three buckets you can actually use at the vet.
Why This Community Exists
I started this community after losing my boy Cooper in 2019. He was given a medication that should never have been prescribed to a Collie. The vet didn't know. I didn't know. And Cooper paid the price.
This site exists because no family should have to go through what we went through. We share our stories, our grief, and most importantly, what we wish we had known.
Read Our Story