The MDR1 test comes back and the report uses language nobody explained to you: homozygous, heterozygous, normal/normal, mut/mut. You are left guessing what it means the next time your dog needs a sedative or an anti-diarrheal. This guide decodes each possible result and tells you what it changes about dosing — which is the only thing the genotype is actually for.
If you have not tested yet, our MDR1 testing guide covers how the cheek-swab test works. This article is about the step after: reading the result.
The Three Possible Results
Every dog has two copies of the MDR1 gene, one from each parent. The test reports the status of both copies, which gives exactly three combinations.
- Normal/Normal (N/N) — also written “+/+” or “wild-type.” Both copies are normal. The dog makes fully functional P-glycoprotein.
- Mutant/Normal (m/N) — also called heterozygous or a carrier. One mutated copy, one normal copy. The dog makes a reduced amount of working P-glycoprotein.
- Mutant/Mutant (m/m) — also called homozygous affected. Both copies are mutated. The dog makes essentially no functional P-glycoprotein.
The words on the report are just longer versions of these three. “Homozygous” means two matching copies; “heterozygous” means two different copies. That is the entire vocabulary.
Why Genotype Matters: The P-Glycoprotein Connection
P-glycoprotein is the pump that keeps certain drugs out of the brain. The MDR1 gene is the instruction manual for building it. The more mutated copies a dog has, the less working pump it builds, and the more freely those drugs reach the central nervous system at doses that would be safe in other dogs.
That is why the genotype is not a label — it is a dosing instruction. It tells your veterinarian how much of the body’s natural drug-defense system your dog actually has.
Normal/Normal: Standard Doses, With One Caveat
A normal/normal herding breed can be dosed like any other dog. The drugs on the problem list pose no special risk because the dog has a full complement of P-glycoprotein.
The one caveat is documentation. If your dog was tested and is clear, write it down and keep the result, because the breed appearance alone will make every new vet cautious. A clear result spares your dog unnecessary dose reductions and lets you treat confidently.
Mutant/Normal (Carrier): The Most Misunderstood Result
This is where the dangerous myths live. Many owners — and some veterinarians — believe that one normal copy makes a dog safe. It does not.
A heterozygous dog makes only a partial amount of functional P-glycoprotein. For many drugs that is enough to behave normally, but for the most sensitive substrates it is not. Carriers can and do have adverse reactions to high-dose ivermectin, loperamide, and several other P-gp drugs. The risk is genuinely lower than for a mutant/mutant dog, but it is not zero, and “lower” is cold comfort in an emergency.
The practical rule veterinary pharmacologists use is that carriers still need dose reductions for the sensitive drugs, just a smaller reduction than a homozygous dog. A frequently cited example is butorphanol: roughly a 25% reduction is suggested for a heterozygous dog versus a 30–50% reduction for a homozygous dog. The direction is identical for both genotypes — start lower than the textbook dose — only the magnitude differs.
Treat a carrier as “at meaningful risk, handle with caution,” not “basically normal.” That single reframing prevents most carrier accidents.
Mutant/Mutant: Maximum Caution, Not Maximum Fear
A homozygous affected dog has no functional P-glycoprotein and is at the highest risk for every drug on the problem list. These are the dogs in which a single Imodium tablet can cause coma, and in which standard sedation doses can be dangerous.
But maximum caution is not the same as maximum fear. A mutant/mutant dog still needs heartworm prevention, still needs pain control after surgery, and still needs anti-nausea drugs when sick. The difference is that every prescription is screened against the problem list first, and P-gp drugs are either avoided or started at a substantially reduced dose. For the full sorted list of what to avoid, reduce, or use freely, see our decoded WSU problem drug list.
What the Genotype Does Not Tell You
Two honest limits are worth knowing. First, the test reports only the common MDR1 deletion; it does not measure how an individual dog will metabolize a drug on a given day, which is influenced by liver function, age, and other medications. Second, severity within a genotype varies — not every mutant/mutant dog reacts identically. The genotype sets the risk category; careful, low-and-slow dosing handles the individual variation on top of it.
This is why the result is a starting point for a conversation with your vet, not a substitute for one.
Turning Your Result Into a Plan
Whatever the report says, do three things. Write the exact genotype on a card for the dog’s tag and your phone. Tell every veterinarian the genotype out loud at the start of each visit rather than trusting the chart. And ask, for any new drug, whether the dose should be adjusted for your dog’s specific status.
A test result sitting in a drawer protects nothing. A genotype that travels with your dog and reaches every prescriber is what actually keeps a herding breed safe — whether the answer was normal/normal, carrier, or mutant/mutant.