Background <p>Lactation ketoacidosis is a rare metabolic complication of breastfeeding, characterised by high anion gap metabolic acidosis due to elevated levels of ketone bodies in the blood or urine. Common symptoms include nausea, vomiting, abdominal pain and general malaise. The typical triggers for lactation ketoacidosis are intensive exercise, a ketogenic diet, reduced food intake or fasting, often associated with an attempt to lose weight.</p> Case presentation <p>We report the case of a 31-year-old woman who had been breastfeeding continuously for two years, including throughout her most recent pregnancy, and had been breastfeeding both children for the past six months, without following any special diet. In November 2023, she was admitted to the emergency department with severe toothache, fever, dysphagia, and odynophagia, leading to the diagnosis of dental cellulitis requiring urgent surgery. In the postoperative period, she developed metabolic acidosis with a high-anion gap, normal lactate, and elevated ketones but without hyperglycaemia, consistent with lactation ketoacidosis. This exceptional presentation was likely triggered by a combination of recent fasting due to dental pain, acute infection, and perioperative metabolic stress. Prompt treatment with bicarbonate, glucose infusion, and enteral hydration led to rapid clinical and biological improvement.</p> Conclusions <p>This case underscores the necessity of a careful pre-anaesthetic history in lactating women, including dietary information, breastfeeding habits and potential stressors such as intercurrent illness. To mitigate the risk of ketoacidosis, preoperative fasting times should be minimised, as recommended. In breastfeeding patients at high risk of metabolic complications, perioperative administration of glucose might be considered. Furthermore, it is crucial to pay close attention to the dental health of pregnant and lactating women to reduce the need for surgical procedures that may lead to such severe complications.</p>