Low blood sodium caused by medicines

Sodium sits mostly in the fluid outside the body's cells and helps regulate blood pressure and nerve and muscle function. MedlinePlus explains that when the sodium in this fluid drops, water moves into cells and makes them swell, and brain cells are especially sensitive to this swelling, which is why many of the symptoms are neurological.

The speed matters as much as the level. A fall over less than 48 hours is more dangerous than one that develops over days or weeks, because in a slow fall the brain cells have time to adjust. People who develop a gradual drop may only feel tired or off balance, which is one reason falls and confusion in older adults are sometimes the first clue.

A 2025 review in the Journal of Clinical Medicine describes drug-induced hyponatraemia as one of the most prevalent but frequently overlooked causes, because comorbidities and polypharmacy make it hard to pin on one medicine. Mechanisms differ by class. Diuretics make the kidney lose sodium, so the body ends up short of salt and water. Many medicines that act on the brain instead cause water to be retained through the syndrome of inappropriate antidiuretic hormone secretion (SIADH), or increase thirst and free water intake.

The review cites a prospective pharmacovigilance programme in which thiazides and thiazide-like drugs such as indapamide and chlortalidone were the leading cause of severe drug-induced hyponatraemia, loop diuretics came next, and SIADH was the second most frequent diagnosis, mainly caused by antidepressants, anticonvulsants and antipsychotics. Other classes, including some cancer, digestive, anti-infective and endocrine medicines, are implicated to a lesser extent. MHRA safety information also describes deaths in children from hyponatraemia after intravenous 0.18% saline with 4% glucose, which is not to be used in children aged 16 or under except in specialist settings under expert supervision.

Older adults are particularly susceptible because ageing reduces the kidney's ability to excrete water, and they often have several conditions and take several medicines. The same review notes that combinations of drugs can raise the risk of electrolyte disorders markedly, sometimes up to ten-fold. A person starting a water tablet in a combined blood pressure tablet, taking an antidepressant and eating little can be affected by all three effects together.

The medicine may not be the only factor. MedlinePlus lists other causes of low sodium such as diarrhoea, vomiting, sweating, heart failure, kidney disease and cirrhosis, and the review stresses that these should be considered alongside the drug. When low sodium appears long after a medicine was started, the authors advise looking for other contributing factors before simply stopping it and weighing benefits against risks.

Common symptoms listed by MedlinePlus include confusion, irritability and restlessness, fatigue, headache, loss of appetite, muscle weakness, spasms or cramps, and nausea and vomiting. In severe cases it can lead to reduced consciousness, hallucinations, seizures or coma.

New confusion, a seizure, severe headache with vomiting or drowsiness after starting or increasing a medicine needs urgent assessment through 999, A&E or NHS 111, rather than waiting for a routine appointment. Milder symptoms with a recent new tablet are worth a prompt call to the GP or pharmacist and a blood test.

Diagnosis starts with a blood sodium test, often as part of a kidney and electrolyte panel, together with blood and urine osmolality and urine sodium to work out the cause. The doctor then examines whether the body is dry, normal or overloaded with fluid, because treatment differs.

MedlinePlus says the cause must be diagnosed and treated, and other treatments depend on the type: fluids through a vein, medicines to relieve symptoms, and limiting water intake. The central step for drug-induced cases is reviewing the medicine list. The review recommends considering whether the suspected drug should be stopped or changed after a benefit-risk assessment, and stresses the role of every healthcare professional in picking it up.

Treatment of significant low sodium is therefore usually supervised in hospital, where the sodium can be rechecked frequently and the cause tracked down.

Because most cases appear within the first weeks, a blood test soon after starting or changing a diuretic, antidepressant or anti-seizure medicine is a reasonable safeguard to ask about, particularly in older people. Follow any fluid advice you are given rather than drinking large volumes to compensate for symptoms, since several of these medicines already make the body hold on to water.

Tell your prescriber about any previous low sodium result, and report suspected reactions through the MHRA Yellow Card scheme.