Between August 1999 and January 2001, moxidectin (Cydectin NF 1%) was administrated orally (0.5mg/kg) or subcutaneously (0.5 - 1.0mg/kg) every 72 hours to 26 dogs with canine demodicosis. Twenty-one dogs were cured, 9 of 12 treated by oral administration (VO) and 12 of 14 medicated subcutaneously (SC). The average treatment period was 22 (VO) and 21 (SC) weeks. Adverse reactions ocurred in 5 dogs (19.2%), 3 after oral treatment and 2 after subcutaneous application. Side effects included lethargy, inappetance/anorexia, adipsia, emesis, dyspnea and facial edema (VO) and lethargy, hypothermia, sensitivity and cutaneous lesions at the site of the moxidectin application (nodule, crusts and purulent exudate). Using Fisher and Student T tests showed there was no relevant statistical difference in the efficacy, rate of adverse reactions and average treatment duration between the oral or subcutaneous treatment protocols. Owners overdosed two of the dogs which caused dysorexia, sialorrhea, mydriasis, and fasciculations and ataxia of the pelvic limbs. This study shows that the veterinarians who use moxidectin should be aware that, although moxidectin is an efficient and low cost drug in Brazil, it may cause systemic (23%) and pharmacodermic (3.8%) adverse reactions, depending on the dose used.