MDR1 Gene Mutation in Dogs: Drug Sensitivity in Herding Breeds

By Tailwisp Editorial TeamUpdated October 17, 20263 min read

Some dogs carry a hidden sensitivity to common medicines, and a simple test can tell you.

What MDR1 is

The MDR1 gene, also called ABCB1, makes a pump protein that moves certain drugs out of the brain. A mutation stops the pump from working well, so drugs build up in the nervous system and can cause toxic effects even at doses that other dogs tolerate.

Breeds commonly affected

• Collies (rough and smooth)
• Australian shepherd (including miniature)
• Shetland sheepdog
• Old English sheepdog
• English shepherd
• Border collie at lower rates
• German shepherd, white Swiss shepherd and others
• Long-haired whippet
• Mixed breeds with herding ancestry

Frequency varies widely between breeds and lines.

Genetic status

• Normal/normal: not at risk
• Mutant/normal (carrier): often mildly affected by some drugs and can pass the gene on
• Mutant/mutant: most sensitive

Testing is a simple cheek swab or blood test. See dog DNA and genetic health testing.

Drugs that need caution

Your vet can check the current lists, but examples include:

• Ivermectin at high doses, such as in horse dewormers
• Loperamide (Imodium)
• Certain chemotherapy drugs, such as vincristine and doxorubicin
• Acepromazine, butorphanol and some sedatives, which may require reduced doses
• Some others, such as emodepside and selamectin at high doses

Many everyday medications are fine, but dosing may need adjusting. See flea, tick and heartworm medications for dogs.

Signs of a reaction

• Drooling and vomiting
• Tremors
• Unsteadiness and disorientation
• Blindness
• Slow heart rate
• Seizures, coma

This is an emergency. See dog poisoning: what to do.

What to do after exposure

1. Call your vet or poison helpline immediately.
2. Bring the product label.
3. Do not wait for signs.
4. Expect hospital monitoring and supportive care.

Preventing reactions

• Test at-risk dogs early, even before first vaccines or surgery
• Use dog-specific products at labeled doses
• Store livestock products securely
• Keep dogs from horse and cow manure in areas where dewormers are used
• Tell every vet, groomer and boarding facility about the result
• Add the result to the microchip record or collar tag

For breeders

Test breeding dogs and avoid producing mutant/mutant puppies where possible. Share results openly.

Living with an affected dog

With good information, these dogs can have normal lives. Keep a card in your wallet and phone with the result.

Questions to ask your vet

Ask which drugs should be avoided, which doses need adjusting and how to handle emergencies.

Common mistakes

• Assuming the breed does not matter
• Using large-animal products
• Not telling new vets
• Skipping the test in mixed breeds

Troubleshooting

• My dog ate horse dewormer paste: call a vet immediately.
• I do not know the breed mix: a test can help.
• My carrier dog needed sedation: ask for adjusted doses.

A simple plan

1. Test.
2. Record the result.
3. Tell everyone who treats your dog.
4. Check before giving drugs.

Real-life examples

A collie owner avoids a dangerous reaction by flagging MDR1 before surgery. A shepherd mix tests as a carrier and gets a custom plan. A farm dog's owner locks up livestock dewormers. Knowledge protects.

Test, tell and check

A small test protects against big risks.

What to put on file

Record the result on your dog's vet file, microchip registry notes, boarding forms and groomer records. Keep a card on your phone listing the genotype and drugs to avoid. If you travel, carry it with your dog's other documents so any emergency clinic can adjust treatment immediately.

Livestock and farm settings

On farms, large-animal dewormers and manure can expose dogs to drugs that affect MDR1 dogs badly. Store products in locked cabinets, clean up dropped doses and keep dogs away from fresh manure after treatment of horses or cattle. If a dose is swallowed, call your vet or a poison helpline at once.

The takeaway

MDR1 mutation causes drug sensitivity. Test herding breeds, tell your vet and check every medication.

This article is general educational information, not veterinary advice. Every dog is different. If your dog is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.