Antidiarrhoeal
Loperamide
Avoid in all dogs with the MDR1 mutation
WSU — Is your pet at risk of an adverse reaction to common drugs? (dose tiers, genotypes, signs)Last verified June 16, 2026
Independent DVM review in progressLoperamide — the active ingredient in over-the-counter Imodium — is one of the clearest 'avoid' drugs for MDR1-affected dogs. At doses used to treat diarrhoea it causes neurological toxicity in dogs with the mutation. WSU advises avoiding it in all dogs with the MDR1 mutation. Because it is a common household medicine, this is a drug owners may reach for without a vet — don't, if your dog could be affected.
“At doses used to treat diarrhea, this drug will cause neurological toxicity in dogs with the MDR1 mutation. This drug should be avoided in all dogs with the MDR1 mutation.”— WSU Veterinary Clinical Pharmacology Laboratory · source
Why MDR1 dogs react to Loperamide
The MDR1 (ABCB1) gene encodes P-glycoprotein, a pump that limits how much of certain drugs reaches the brain and helps the body excrete them. Dogs with the MDR1 mutation cannot make a fully functional pump, so these drugs accumulate at the blood–brain barrier and cause neurological toxicity.
Signs of toxicity to know
WSU describes severe adverse reactions in affected dogs as tremors, disorientation, blindness, lack of muscle control, and death. If your dog shows these signs after a medication, treat it as an emergency and contact a veterinarian or emergency clinic immediately — this is not a wait-and-see situation, and it is not a question for a website.
Because Loperamideis an avoid-entirely drug, it's worth knowing the plan if your dog is positive. See what to do if your dog tests positive.
Check Loperamideagainst your dog's breed
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The breed sets the baseline likelihood of the MDR1 mutation. Only a DNA test confirms an individual dog's genotype.
This is general information, not veterinary advice for your dog. It does not diagnose or prescribe. Always discuss any medication decision with your veterinarian before acting — they know your dog's full picture, including its MDR1 status if it has been tested. See our disclaimer and how we research.