Blood pressure medications taken by millions of Britons may increase the risk of a dangerous drop in blood sodium, new research suggests.
The condition, called hyponatremia, can cause confusion, fatigue, headaches, nausea and, in severe cases, seizures.
A large Swedish study, published in JAMA Network Open, followed more than 159,000 adults who had started treatment with either thiazide diuretics – often called ‘water pills’ – or calcium channel blockers.
Patients were tracked for several years to see who developed low sodium levels.
The study found that younger adults had almost no risk. But women over 80 were particularly vulnerable.
About three in 100 older women on thiazides developed dangerously low sodium levels, compared with just over one in 100 taking other blood pressure medications. Men in the same age group were much less affected.
Thiazide and thiazide–like diuretics, including hydrochlorothiazide, chlorthalidone, and indapamide, are commonly prescribed in the UK to treat high blood pressure and heart failure.
They work by helping the body remove excess fluid and by widening blood vessels to lower blood pressure.
About three in 100 older women on thiazides developed dangerously low sodium levels
For older women, the Number Needed to Harm (NNH) – the number of patients who need to take the drug for one to be harmed – was just 53, showing this is not a rare side effect in this group.
High blood pressure, or hypertension, affects around a third of adults and increases the risk of heart attacks, strokes, kidney problems and dementia.
ion, affects around a third of adults and increases the risk of heart attacks, strokes, kidney problems and dementia.
Blood pressure can be checked at home or at many pharmacies, and is especially advised for adults aged 40 and over.
Doctors say the findings could change how blood pressure treatments are prescribed.
‘Older adults, especially women, should have their sodium levels monitored closely if they start thiazides,’ the researchers advise.
In some cases, alternative medications, like calcium channel blockers, may be safer.
Experts recommend regular blood tests, particularly in the first few months of treatment. Patients should report any confusion, dizziness, or unusual tiredness to their GP.
Thiazides remain effective and widely used, with over 14.6 million prescriptions dispensed in England in 2018 alone.
UK guidelines, including NICE recommendations, prefer thiazide–like medications such as indapamide and chlortalidone because of stronger evidence for heart benefits, although bendroflumethiazide is still commonly used.
This research highlights the importance of tailoring blood pressure treatment to individual patients, particularly older adults who may be at higher risk of side effects.
