Breed Health Guide • Reviewed by Dr. Tiffany Delacruz, DVM
MDR1 Gene Mutation (Multi-Drug Sensitivity) in Australian Shepherds
Also known as: MDR1
The MDR1 gene mutation is a specific genetic abnormality common in Australian Shepherds and related herding breeds that causes life-threatening reactions to certain commonly prescribed medications. The most famous example is ivermectin sensitivity — but the list includes many drugs a vet might use without thinking. Testing your Aussie for MDR1 status before any sedation or unusual medication is genuinely lifesaving information.
Why Australian Shepherds are predisposed to mdr1 gene mutation (multi-drug sensitivity)
Genetic, inherited as autosomal recessive. The mutation originated in a single dog several centuries ago and spread through working herding breed populations. About 50% of Australian Shepherds carry at least one copy (heterozygous); ~15-20% are homozygous (two copies — fully affected). Other affected breeds: Collies, Australian Shepherds, Long-Haired Whippets, McNabs, Old English Sheepdogs, Shelties, Border Collies, and German Shepherds.
What you'll see at home
- Severe neurologic signs after exposure to a problematic drug
- Disorientation, ataxia, tremors
- Excessive drooling
- Vomiting
- Coma in severe cases
- Death without prompt intervention in severe overdose
- Symptoms typically appear within hours of exposure
Red flags — go to an emergency vet
- ⚠ Any acute neurologic signs (disorientation, tremors, ataxia) — possible MDR1 drug reaction
- ⚠ Recent ingestion of horse-strength ivermectin or other livestock dewormers
- ⚠ Loperamide (Imodium) ingestion (do not give Imodium to herding breeds)
- ⚠ Any unusual reaction after a routine medication or sedation
- ⚠ Symptoms of any acute drug reaction with neurologic features
How vets diagnose mdr1 gene mutation (multi-drug sensitivity)
Genetic test for the MDR1 mutation — available through Washington State University Veterinary Clinical Pharmacology Laboratory and other testing labs. Single test, lifetime result, ~$60-100. Every Australian Shepherd should be tested before any sedation or potentially problematic medication.
Treatment options
No treatment for the genetic mutation itself. The 'treatment' is avoidance of problematic drugs. Drug list includes: ivermectin (at horse-deworming doses; heartworm prevention doses are safe), loperamide (Imodium), acepromazine, butorphanol (some uses), erythromycin, vincristine, doxorubicin, certain chemotherapy drugs, and several others. For active drug reaction: supportive care (IV fluids, anti-seizure medications, lipid emulsion therapy in some cases).
Living with a Australian Shepherd who has mdr1 gene mutation (multi-drug sensitivity)
- 1 Test your Australian Shepherd for MDR1 BEFORE any sedation or unusual medication — single test, lifetime answer
- 2 Carry an MDR1 medical card identifying your dog as positive (if tested positive)
- 3 Tell every vet, including emergency vets, about MDR1 status
- 4 Use only veterinary-prescribed heartworm prevention at standard doses (these are safe — the ivermectin dose is far below the threshold)
- 5 NEVER give livestock-strength ivermectin to an Aussie
- 6 NEVER give loperamide (Imodium) to herding breeds — major MDR1 substrate
- 7 Discuss any new prescription with your vet specifically about MDR1 implications
- 8 Print and share the WSU Clinical Pharmacology drug list with anyone caring for your Aussie
Can RexVet help with this online?
RexVet is well-suited for: pre-procedure MDR1 testing recommendations, reviewing whether a specific medication is safe for your Aussie's MDR1 status, refilling MDR1-safe medications, and general Australian Shepherd preventive care guidance.
Start a $64.99 video visit →We cannot run MDR1 genetic tests by video (saliva or cheek-swab kits are widely available). Active drug reactions are emergencies requiring in-person stabilization. Initial diagnosis of an acute reaction needs in-person evaluation.
Prognosis — what to expect
Excellent — the genetic mutation itself causes no symptoms unless the dog encounters a problematic drug. Lifetime drug avoidance plus MDR1 awareness means an MDR1-positive Aussie can live a completely normal life. Catastrophic outcomes are entirely preventable with testing and awareness.
Frequently asked questions
Frequently asked questions
Should I test my Australian Shepherd for MDR1?
Yes — every Aussie should be tested. The test is a single cheek swab, costs ~$60-100, gives a lifetime answer, and the information is critical for any future sedation or unusual prescription. Even heterozygous (carrier) dogs can have sensitivity to some drugs at high doses. Knowing is genuinely lifesaving information.
Can my MDR1-positive Australian Shepherd still take heartworm prevention?
Yes — standard heartworm prevention doses of ivermectin are far below the threshold that causes problems in MDR1-affected dogs. The doses in milbemycin or selamectin products are also safe. The danger comes from livestock-strength ivermectin (which has 100x the dose of heartworm prevention) and from a few specific other drugs. Always use only vet-prescribed heartworm prevention.
What medications should I avoid with my MDR1-positive Aussie?
The most common problematic drugs are: livestock-strength ivermectin, loperamide (Imodium), acepromazine, butorphanol in some uses, and certain chemotherapy drugs. The Washington State University Veterinary Clinical Pharmacology Laboratory maintains the definitive list — print it and share with any vet treating your dog.
Other helpful RexVet resources for Australian Shepherds parents
Sources
- WSU Veterinary Clinical Pharmacology Laboratory — MDR1
- ACVIM — MDR1 Drug Sensitivities
- AKC Australian Shepherd Breed Page
Last fact-checked: 2026-06-04. Reviewed by Dr. Tiffany Delacruz, DVM.
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