Acute Intermittent Porphyria Mimicking Severe Coprostasis in a Patient on Opioid Maintenance Therapy.

Introduction: Acute intermittent porphyria (AIP) is a rare hereditary metabolic disorder in haem biosynthesis that presents with severe abdominal pain. Attacks may be triggered by caloric deprivation. Diagnostic delay is common and reflects non-specific clinical presentations, the rarity of the disease and cognitive biases in clinical decision-making. Case description: A 30-year-old woman with opioid and benzodiazepine use disorder presented with abdominal pain and constipation and was initially treated for opioid-induced constipation. Persistent symptoms and hyponatraemia prompted further evaluation, revealing markedly elevated urinary porphyrin precursors. She was diagnosed with AIP and treated with intravenous hemin and high-carbohydrate therapy. Discussion: This case illustrates the diagnostic challenge of AIP when plausible alternative explanations such as opioid-induced constipation are present. Persistent symptoms and hyponatraemia due to SIADH served as key clues prompting diagnostic reassessment. Diagnostic overshadowing may have contributed to delayed recognition in the context of substance use disorder. Concurrent severe malnutrition with caloric deprivation likely triggered the attack via accumulation of neurotoxic haem precursors, underscoring the importance of diagnostic flexibility in atypical or non-resolving cases. Conclusion: This case underscores the need to consider AIP in patients with severe abdominal pain and atypical clinical courses. LEARNING POINTS: Acute intermittent porphyria (AIP) can present as opioid-induced coprostasis, increasing the risk of diagnostic anchoring and delay.Coexisting factors (opioid therapy, psychiatric comorbidity, malnutrition) may act simultaneously as triggers, mimics and sources of cognitive bias.Reconsidering the initial diagnosis in atypical or non-resolving cases is essential to counter cognitive bias and improve outcomes in rare but treatable conditions.