MDR1 Gene Mutation in Dogs: Drug Sensitivity and Why Testing Matters

The MDR1 mutation — also known as the ABCB1 gene mutation — affects the blood-brain barrier in dogs, allowing certain drugs that are normally excluded from the brain to penetrate at toxic levels. In affected dogs, medications that are completely safe for most dogs can cause tremors, seizures, coma, and death. Testing is simple, inexpensive, and essential for all breeds that carry this mutation.

What Is the MDR1 Gene?

MDR1 (multidrug resistance gene 1) produces P-glycoprotein, a protein that pumps certain drugs out of brain cells and prevents them from reaching toxic concentrations. Dogs with the MDR1 mutation produce a non-functional or absent P-glycoprotein, leaving the brain unprotected against certain drug classes.

Which Drugs Are Dangerous for MDR1-Affected Dogs?

Ivermectin: An antiparasitic used in heartworm prevention. Standard veterinary doses are safe for all dogs, but certain doses used for mange treatment, or the concentrations in some livestock products, can be fatal in MDR1-affected dogs. The phrase "white feet, don't treat" emerged from Collie breeders warning each other about ivermectin toxicity.

Other problematic drugs include:

• Loperamide (Imodium) — can cause severe neurological symptoms
• Acepromazine — sedative that causes prolonged or deepened sedation
• Butorphanol — narcotic analgesic
• Vincristine, doxorubicin — chemotherapy agents
• Certain macrocyclic lactones (milbemycin, selamectin at high doses)
• Some antifungals

Breeds Most Affected

The mutation originated in a single ancestor of herding breeds and is now widespread in:

• Rough and Smooth Collies
• Shetland Sheepdogs (Shelties)
• Australian Shepherds
• Border Collies
• English Shepherds
• Old English Sheepdogs
• German Shepherds (lower prevalence)
• McNabs

Approximately 75% of Rough Collies are affected (homozygous or heterozygous). Australian Shepherd prevalence is around 50%.

MDR1 Test Results

MDR1 Normal/Normal (N/N): Not affected. Normal P-glycoprotein function. MDR1 Affected/Normal (MDR1/N): One copy of mutation. Moderate sensitivity — intermediate drug accumulation in brain. MDR1 Affected/Affected (MDR1/MDR1): Two copies — severely affected. Maximum drug sensitivity.

Both heterozygous and homozygous affected dogs can have serious reactions, though severity varies.

What to Tell Your Vet

If your dog tests positive for MDR1, give your vet a copy of the test results at every appointment. A wallet card or note in the dog's file stating "MDR1 mutation — avoid ivermectin, loperamide, acepromazine, butorphanol" can save your dog's life in an emergency.