Recreational drug use is a significant societal issue and remains a clinical challenge in emergency and critical care departments. We report on a 19-year-old woman admitted to hospital semiconscious and with severe hyponatraemia. Urinalysis was positive for methamphetamine and supported a diagnosis of hyponatraemia related to ecstasy use together with a syndrome of inappropriate antidiuretic hormone secretion (SIADH). The woman was transferred to an intensive care unit, where a hypertonic saline infusion was started. Three hours postadmission she developed polyuria. Follow-up urinalysis at this point was consistent with water intoxication. This case is a reminder that hyponatraemia is a potentially fatal complication after the ingestion of 3,4-methylenedioxymethamphetamine, illustrates the sequential nature of an SIADH and water intoxication and highlights the importance of considering the sequence of onset of hyponatraemia, as the patient may be admitted at any stage.