Recurrent Acute-on-Chronic Hyponatraemia: A Diagnostic Odyssey Involving Drug-Induced Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH), Chronic Cannabis Use, and Occult Lung Adenocarcinoma.

Hyponatraemia is a common clinical presentation and one of the most frequently encountered electrolyte abnormalities in hospitalised patients, with syndrome of inappropriate antidiuretic hormone secretion (SIADH) representing a major underlying cause. Clinical manifestations are variable, with confusion being a common presenting feature. While hyponatraemia is often readily identified and managed, recurrent or complex presentations may reveal multiple coexisting aetiologies, including malignancy, medication-related causes, substance misuse, and neuropsychiatric disorders. Establishing the underlying cause can be challenging, particularly when initial investigations appear to provide a satisfactory explanation for the presenting episode. This case highlights the importance of ongoing diagnostic reassessment in patients with recurrent hyponatraemia and confusion, as well as the need to consider multifactorial contributors when the clinical course is not fully explained by a single diagnosis.