Multidrug Resistance 1 is inherited as an autosomal incomplete dominant disease. Dogs only need to inherit one copy of the MDR1 mutation to be at risk for sensitivity of certain drugs. Dogs with 2 copies of MDR1 can have more severe reactions. There are 3 possible genotypes reported for Multidrug Resistance 1: CLEAR (those having 2 copies of the normal allele and appear to be normal, not affected by MDR1) CARRIER (those having 1 copy of the normal allele and 1 copy of the mutation, increased risk of drug sensitivity) AFFECTED (those having 2 copies of the mutation and at risk for adverse drug reactions). Dogs with one copy of the MDR1 mutation can have some drug sensitivity. Dogs with two copies of this mutation can experience more serious neurologic symptoms like excess salivation, tremors, anorexia, blindness and possibly death. The following is a list of drugs known to cause reactions: Acepromazine (tranquilizer), butorphanol (pain control), doramectin, doxorubicin, emodepside, erythromycin, ivermectin (found in heartworm medications), loperamide (anti-diarrheal agent), milbemycin, moxidectin, paclitaxel, rifampin, selamectin, vinblastine (anti-cancer agent), vincristine and vinorelbine *You should inform your veterinarian if your dog is a MDR1-carrier or MDR1-affected so they can give your dog the adjusted dosage or an alternative medication. Other medications not listed may also cause a reaction. Breeds, crosses and mixes of breeds that are more commonly affected by MDR1 are: Australian Shepherd, Border Collie, Collie, English Shepherd, German Shepherd Dog, Longhaired Whippet, McNab Shepherd, Miniature Australian Shepherd, Old English Sheepdog, Shetland Sheepdog, Shiloh Shepherd, Wäller, White Shepherd Dog (Note: test does not determine breed)
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