Twenty-two dogs diagnosed with generalized demodicosis were treated with milbemycin oxime (MO) because of poor response to previous therapies or because the dog was a breed known to be susceptible to ivermectin toxicosis. Fifteen of the 22 dogs were herding breeds. Doses of MO ranged from 1.0 to 2.2 mg kg−1 day−1 per os. Cheek swab samples were obtained in order to determine each dog's ABCB1 genotype. Adverse drug reactions were recorded for each dog by the owners and/or veterinarians. The ABCB1-1Δ genotype was significantly associated with the development of an adverse reaction (neurological toxicity) after treatment with MO. None of the 19 dogs with the wild-type ABCB1 allele experienced adverse reactions, whereas two dogs homozygous for the ABCB1-1Δ mutation developed ataxia. Assessing the ABCB1-1Δ genotype prior to MO administration may prevent neurological toxicity in these patients.