MDR1 stands for Multi-Drug Resistance gene 1, also called ABCB1. It's a perfectly normal gene that every dog is supposed to have. The story that matters for medication safety isn't the gene itself — it's what happens when a dog inherits a mutated copy of it.
What the gene normally does
The MDR1 gene carries the instructions for building a protein called P-glycoprotein. Think of P-glycoprotein as a tiny pump that sits in the walls of certain tissues — most importantly the barrier between the bloodstream and the brain. According to the Washington State University Veterinary Clinical Pharmacology Laboratory, P-glycoprotein “plays an important role in limiting drug distribution to the brain and in enhancing the excretion of many drugs.”
In other words, when a healthy dog takes a drug that P-glycoprotein recognises, the pump keeps most of that drug out of the brain and helps the body clear it. The drug does its job in the body without flooding the central nervous system.
What the mutation changes
A dog with the MDR1 mutation can't build a fully working pump. When such a dog takes one of these drugs, P-glycoprotein isn't there to keep it out of the brain — so the drug accumulates at the blood–brain barrier and crosses into the central nervous system at levels a normal dog would never reach. That accumulation is what causes the neurological toxicity associated with MDR1.
WSU describes the resulting severe reactions as tremors, disorientation, blindness, lack of muscle control, and deathin the worst cases. It's important to be clear-eyed about this without catastrophising: these reactions happen when an affected dog is given an affected drug at a problem dose. They are not a constant state of fragility, and they are entirely avoidable once the drug list is known.
Three genotypes, three risk levels
Every dog has two copies of the MDR1 gene — one from each parent. That gives three possible combinations, and WSU classifies dogs accordingly:
- Normal / normal— both copies are normal. The dog isn't at increased risk from these drugs at normal doses.
- Mutant / normal (heterozygous) — one mutated copy. An intermediate risk: WSU notes these dogs can also experience toxicity and should receive reduced doses of affected drugs.
- Mutant / mutant (homozygous)— both copies mutated. The highest-risk genotype, and the one where WSU's strongest cautions apply.
Crucially, you cannot tell a dog's genotype from how it looks or behaves. The only way to know is a DNA test.
Which dogs carry it
The mutation is concentrated in herding breeds and a few sighthounds. WSU's published frequency table puts the Collie highest — roughly 70% carry at least one copy — followed by the Australian Shepherd and Long-haired Whippet around 50%, and a long tail of other breeds at lower frequencies, including the Shetland Sheepdog, German Shepherd, and herding-breed and mixed-breed crosses. Notably, despite the similar name, the Border Collie sits below 5% — far lower than the Collie.
These are population frequencies, not individual predictions. A 70% breed frequency means most Collies carry the mutation, but plenty don't — which is exactly why a DNA test, rather than the breed name, is what settles the question for your specific dog.
Why it matters
MDR1 matters because the drugs it affects are common ones: the antiparasitic ivermectin, the over-the-counter antidiarrhoeal loperamide (Imodium), several sedatives and pre-anaesthetics, and a number of chemotherapy agents. A dog could encounter any of these during a routine vet visit, a surgery, or even an owner's well-meaning attempt to settle an upset stomach.
The good news is that MDR1 is one of the most manageable genetic conditions a dog can have. The affected drugs are known and finite. For the antiparasitics, the everyday heartworm-prevention dose is safe — only the much higher treatment doses are a problem. And for nearly everything else, the answer is simply to tell your vet your dog's status so they choose the dose or the alternative accordingly.
That's the whole job: know the list, flag it with your vet, and — if you want certainty about your individual dog — test. The rest of this site is built to make those three steps easy.