Antibiotic prophylaxis in dogs undergoing surgical procedures frequently involves the administration of a product without a veterinary licence. Two drugs commonly used for this purpose are the clavulanate amoxicillin Augmentin and the cefuroxime Zinacef. This prospective observational study aims to compare the incidence of adverse events associated with these two antibiotics in a clinical setting. The authors hypothesised that a higher incidence of adverse effects would be observed with Augmentin. Sixty-five dogs were included in the study and adverse events were recorded using a modified scoring system. A significantly higher incidence of adverse events to Augmentin (8/22; 36 per cent) was observed compared with Zinacef (1/43; 2 per cent) (P=0.0003). The majority of these adverse events involved cutaneous signs and/or hypotension. These findings might be taken into consideration when selecting one of these intravenous antibiotics for prophylaxis in anaesthetised dogs undergoing surgery.