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Some dog breeds are more sensitive to certain drugs than other breeds. Collies and related breeds, for instance, can have adverse reactions to drugs such as ivermectin and loperamide (Imodium).

Drug sensitivities result from a mutation in the multi-drug resistance gene (MDR1). This gene encodes a protein, P-glycoprotein that is responsible for pumping many drugs and other toxins out of the brain. Dogs with the mutant gene cannot pump some drugs out of the brain as a normal dog would, which may result in abnormal neurologic signs. The result may be an illness requiring an extended hospital stay - or even death.

Approximately three of every four Collies in the United States have the mutant MDR1 gene. The frequency is about the same in France and Australia, so it is likely that most Collies worldwide have the mutation.

Many different drugs and drug classes have been reported to cause problems in Collies and other herding breed dogs that carry the MDR1 mutation. We suggest that you link to the Washington State University web site which has a list of drugs that have been researched because information on these drugs changes often.

The most common drugs known to cause neurological signs are listed below. We suggest that these drugs be avoided completely unless you are absolutely sure the amount to be administered is in the "safe" dosage zone.

- Invermectin (antiparasitic agent).
- Acepromazine (tranquilizer and pre-anesthetic agent).
- Butorphanol (analgesic and pre-anesthetic agent).
- Erythromycin
- Loperamide
- Selamectin, milbemycin, and moxidectin (antaparasitic agents).
- Vincristine, Vinblastine, Doxorubicin (chemotherapy agents).