MDR1 Gene Mutation Drug Sensitivity Checker for Dogs
The MDR1 (ABCB1) gene mutation makes certain drugs cross into the brain and accumulate there instead of being pumped back out. Look up a breed or a drug below to check the risk tier - then confirm with genetic testing before dosing.
This is a screening reference, not a substitute for genetic testing.
Standard heartworm preventive doses of ivermectin and milbemycin remain safe even in confirmed MDR1 mutant/mutant dogs. Toxicosis is a dose-dependent phenomenon seen at much higher antiparasitic/mange-treatment doses, not prevention doses.
Part A - Breed lookup
Part B - Drug lookup
How to use this tool
Why MDR1 matters
The MDR1 gene encodes P-glycoprotein, an efflux pump at the blood-brain barrier that normally pumps certain drugs back out of the brain. A deletion mutation in this gene leaves the pump non-functional, so affected dogs' brains accumulate the drug and become sensitive at doses that are perfectly safe in normal dogs.
Which breeds are affected
Herding breeds descended from a common ancestor - Collies, Australian Shepherds, Shetland Sheepdogs, and related breeds - carry the highest reported prevalence. Inheritance is incompletely dominant: homozygous mutant/mutant dogs are most severely affected, while heterozygous mutant/normal dogs are partially affected but still at risk.
Heartworm prevention is still safe
Standard monthly heartworm preventive doses of ivermectin and milbemycin remain safe even in confirmed mutant/mutant dogs. Toxicosis is dose-dependent and is seen at the much higher doses used for mange or heavy parasite treatment - not at prevention doses.
Confirming status
Coat colour and appearance don't reliably predict genotype. A cheek swab or blood genetic test - such as the one offered by Washington State University's Veterinary Clinical Pharmacology Laboratory - is the only way to confirm MDR1 status before prescribing a P-glycoprotein substrate drug.
Frequently asked questions
What is the MDR1 gene mutation?
The ABCB1/MDR1 gene encodes P-glycoprotein, an efflux pump at the blood-brain barrier that normally pumps certain drugs back out of the brain. A deletion mutation makes this pump non-functional, so affected dogs' brains accumulate the drug and become sensitive to normal doses.
Which breeds are affected?
Herding breeds descended from a common ancestor carry the highest reported prevalence - Rough and Smooth Collies, Australian Shepherds (standard and Miniature), Shetland Sheepdogs, Long-haired Whippets, Old English Sheepdogs, Silken Windhounds, McNabs, and English Shepherds. German Shepherds, Border Collies, and herding-breed mixes have been reported but at lower prevalence. Inheritance is incompletely dominant: homozygous mutant/mutant dogs are most severely affected, heterozygous mutant/normal dogs are partially affected but still at risk.
Is heartworm prevention safe in MDR1-mutant dogs?
Yes. Standard monthly heartworm preventive doses of ivermectin and milbemycin are safe even in confirmed mutant/mutant dogs. Toxicosis is a dose-dependent phenomenon seen at the much higher doses used for mange or heavy parasite treatment, not at prevention doses.
How do I test for the MDR1 mutation?
A cheek swab or blood genetic test confirms MDR1 status. Washington State University's Veterinary Clinical Pharmacology Laboratory offers the widely-used commercial test. Testing is recommended for any herding breed or herding mix before prescribing a P-glycoprotein substrate drug.