Coat color cannot tell you whether a dog carries the MDR1 (ABCB1) variant that makes some medications dangerous. White feet, a collie-like face, or a herding-dog look may suggest a breed background, but none of them establishes or rules out the mutation. Only genetic testing identifies it, and only a veterinarian who knows the dog’s genotype can judge whether a specific drug, at a specific dose, is appropriate.
What “white feet don’t treat” means
“White feet don’t treat” is an old piece of visual shorthand among dog owners and some breeders: a dog that looks like a herding breed, especially one with white markings, was assumed to be the dog to worry about with certain drugs. That assumption does not hold up. Coat pattern is not a reliable indicator of MDR1 status, and the mutation is not confined to dogs that look like herding breeds. A dog with no obvious herding ancestry may carry it, and a dog that looks like a collie may not.
The practical lesson is simple. Do not use appearance to decide whether a medication is safe, and do not use appearance to decide whether a test is worth doing. If your dog has any herding-breed ancestry, a mixed background, or simply an unknown pedigree, the question of MDR1 status is a fair one to raise with your veterinarian.
What MDR1 and P-glycoprotein do
MDR1 is the older, still common name for the ABCB1 gene. The gene encodes P-glycoprotein (P-gp), a transporter protein found at the blood-brain barrier and in other tissues. In normal dogs, P-gp acts as a gatekeeper: it pumps many drugs back out of the brain and helps govern how the body disposes of them.
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The mutation that the American Animal Hospital Association (AAHA) describes is a four-base-pair deletion in the gene, identified in 2001. The deletion interrupts production of the protein, and the resulting transporter can be nonfunctional. When P-gp does not work, some drugs that would normally be kept out of the nervous system can reach or accumulate in the brain and cause neurologic toxicity. The effect is not uniform. It depends on the drug, the dose, and whether the dog carries one copy of the variant or two.
Which dogs may be affected
AAHA’s article “White feet don’t treat: Considerations for dogs with MDR1 mutations,” by Kate Boatright, VMD, published December 31, 2024, reports the prevalence estimates below. These are population-level figures from the article, not a probability that any individual dog carries the variant.
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| Group | Reported prevalence | Source and date |
|---|---|---|
| Collies | Up to 70% | AAHA, 2024 |
| Long-haired whippets | 65% | AAHA, 2024 |
| Other herding breeds | Up to 50% | AAHA, 2024 |
| Silken Windhounds | 30% | AAHA, 2024 |
| McNabs | 30% | AAHA, 2024 |
| English shepherds | 15% | AAHA, 2024 |
| German shepherds | 10% | AAHA, 2024 |
| Mixed-breed dogs | Up to 10% | AAHA, 2024 |
Cornell University College of Veterinary Medicine’s Richard P. Riney Canine Health Center lists collies, Australian shepherds, American shepherds, German shepherds, Shetland sheepdogs, and Old English sheepdogs among the herding breeds commonly affected. The breed list tells you where the variant is concentrated; it does not tell you whether a given dog has it. Testing is the only way to know.
Signs of toxicity
Reported signs of toxicity in dogs with the variant include:
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- Weakness
- Ataxia, meaning uncoordinated movement
- Tremors
- Seizures
- Blindness
- Vomiting (listed by Cornell)
- Death, in severe cases
Cornell explains that dogs with two copies of the variant may be more severely affected, while dogs with one copy can still experience toxicity, generally with less severe effects. These signs are not specific to MDR1, so they should not be used as a home checklist. If your dog develops any of them after receiving a medication, contact your veterinarian or an emergency clinic promptly and tell them about the drug and any known genotype.
Medications: the dose and the drug both matter
This is where the most common confusion happens. AAHA states that ivermectin and related drugs are safe at the doses used for heartworm prevention, but may become toxic at the much higher doses used to treat mange. The same distinction runs through the rest of the guidance: a drug’s safety profile is tied to its dose and use, not to the drug name alone.
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AAHA also reports that manufacturers have tested products containing alfaxolaner, fluralaner, ivermectin, milbemycin, moxidectin, sarolaner, and selamectin, and that these are considered safe at FDA-approved doses for dogs with MDR1 mutations. Cornell likewise states that heartworm preventives containing ivermectin are safe at low, FDA-approved doses. These statements apply to the approved, labeled uses. They do not extend to off-label dosing, high-dose treatment, or combinations that a veterinarian has not reviewed.
Other drugs that sources say warrant discussion before use include:
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- Loperamide (Imodium): Cornell identifies this as a drug to avoid entirely in MDR1-sensitive dogs.
- Acepromazine and butorphanol: AAHA lists these among medications to discuss with the veterinarian.
- Some chemotherapy agents, antiemetics, and cyclosporine: named as warranting review.
- Drug combinations: Cornell notes that some combinations can raise drug concentrations, so a list of everything a dog takes matters as much as any single product.
This list is not exhaustive. For a current, drug-by-drug review, Washington State University’s problem-medication resource is the appropriate reference, and it should be checked alongside your veterinarian’s advice.
Cornell’s MDR1 resource, “Drug sensitivity: MDR1,” gives the rule owners should follow: “Speak with your veterinarian before changing any drug dosages or discontinuing their use.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Testing and what to do next
Cornell calls genetic testing the most helpful diagnostic tool for MDR1. AAHA identifies Washington State University’s Program for Individualized Medicine (PrIMe) as a test provider and notes that samples can be collected by cheek swab or blood draw. Confirm current ordering details, sample requirements, turnaround, and price directly with WSU before you send a sample, since these can change.
- Order a genetic test that specifically identifies the MDR1/ABCB1 variant, and confirm that it does so before you buy it.
- Collect the sample as the testing provider instructs, and keep the result with your dog’s records.
- Give a copy of the result to your veterinarian, including any result that says the dog does not carry the variant, so the treatment plan reflects the dog’s actual genotype.
- Before any new prescription, preventive, anesthetic, or antidiarrheal, or any change in dose, ask your veterinarian to review it against the result. Tell them about every product your dog receives, including over-the-counter medications.
- Do not stop or change a medication on your own. If you have concerns about a current drug, call your veterinarian first.
A consumer DNA or health-screening kit is not a substitute for a test that is specifically designed for MDR1. Check that the exact product covers the MDR1/ABCB1 variant before relying on it.
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