New research indicates that blood pressure medications commonly used by millions across the UK might elevate the risk of experiencing a significant drop in blood sodium levels.

This condition, known as hyponatremia, can lead to symptoms such as confusion, fatigue, headaches, nausea, and in severe instances, seizures.

According to a substantial Swedish study published in JAMA Network Open, more than 159,000 adults who began treatment with either thiazide diuretics—often referred to as ‘water pills’—or calcium channel blockers were observed.

The study monitored these patients over several years to identify those who developed low sodium levels.

Findings revealed that younger adults faced minimal risk. However, women aged over 80 were notably susceptible.

Approximately three out of every 100 older women on thiazide diuretics experienced dangerously low sodium levels, compared to just over one in 100 taking other blood pressure medications. Men in the same age bracket were significantly 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.