BACKGROUND: Anorexia nervosa has the highest mortality rate of any psychiatric disorder, and purging behaviors can cause a fatal electrolyte and acid-base imbalance. Routine laboratory testing during inpatient care is essential because these patients often provide inaccurate information about their diet and purging behaviors. However, blood gas analysis for an acid-base evaluation is rarely performed in the psychiatric setting because psychiatrists are not accustomed to evaluating the results. This case highlights severe metabolic acidosis caused by excessive laxative use during inpatient care. CASE PRESENTATION: A 62-year-old female patient was admitted with suspected anorexia nervosa. She had stringently controlled her weight since her 20s, initiating laxative use in her 30s. In her 60s, she was referred to our hospital for suspected anorexia nervosa. On admission, her body mass index was 11.0 kg/m². Persistent complaints of severe constipation prompted an escalation in her laxative regimen. Abdominal radiographs demonstrated marked gas retention, corroborating her complaint. On hospital day 84, she abruptly lost consciousness with watery fecal incontinence. Her venous blood gas analysis demonstrated hyperchloremic metabolic acidosis caused by bicarbonate loss secondary to diarrhea. After regaining consciousness, she explained that watery stools were her typical bowel pattern. Discontinuation of laxatives ameliorated her condition. CONCLUSION: In this case, venous blood gas analysis was pivotal in detecting metabolic acidosis resulting from excessive laxative use. Physiological changes due to purging can be better evaluated by incorporating venous blood gas analysis into routine assessment. Further clinical studies are required to validate its utility in managing anorexia nervosa.