Taking HRT drugs for lower than a yr places ladies susceptible to lethal blood clots, research finds
Hormone replacement therapy pills could increase the risk of potentially fatal blood clots by 60 per cent, a major study has warned.
The threat applies to those who use the pills for less than a year or take the lowest dose.
Meanwhile, those who take high doses of HRT face a 40 per cent higher risk of heart attack and a 50 per cent higher risk of stroke. Women on HRT for more than five years could see the risk of both rise to up to 80 per cent.
However, those who use hormonal skin patches, gels and sprays – known as transdermal treatment – do not face the same danger, researchers concluded.
Previous studies have linked the drugs – taken by around two million British women – to blood clots, stroke and heart attacks. But few have identified a dosage that carries the biggest risk.
The authors of the new study, from North Zealand hospital in Denmark, say the data highlights the importance of choosing a specific type, dose and duration of treatment for patients vulnerable to clots.
Researchers analysed health registry data on more than one million Danish women aged between 50 and 69 between 2003 and 2021.
Hormone replacement therapy pills could increase the risk of potentially fatal blood clots by 60 per cent
Some 9,807 women had a serious blood clot, known medically as venous thromboembolism. A further 18,460 had a stroke and 11,974 a heart attack.
Each woman who suffered one of these conditions was matched by age to five women who had not, with researchers statistically adjusting for a range of other important factors such as medical history that could influence clot risk.
The women were then analysed for evidence of HRT prescriptions, including dosage and duration of treatment.
Scientists looked for associations between those who’d taken HRT and had the life-threatening conditions.
Overall, oral HRT was associated with a 60 per cent higher rate of blood clots, with increased rates seen for both oestrogen-only treatment and those combining oestrogen with progestogen.
The problem happens when a gel-like clump of blood cells and proteins form inside a vein, blocking the flow of blood and oxygen around the body.
A clot in a deep vein can break loose and travel to the lungs, causing a potentially fatal pulmonary embolism.
Stroke and heart attack involve clots in the circulation that can cut off the supply of blood and oxygen to the brain or heart.
Studies have shown that when swallowed, oestrogen increases the production of blood-clotting proteins in the liver.
Treatments that enter the bloodstream through the skin, however, bypass the liver.
The latest findings showed that women taking HRT for under a year faced the same raised risk of blood clots as those using the treatment for more than five years.
Similarly, even taking a daily dose below 1mg – defined by study authors as low – was found to significantly increase blood clot risk.
Meanwhile, staying on a daily oral dose above 1mg for more than five years was linked to an 80 per cent higher risk of both heart attack and stroke.
For more than a year of high dose treatment, the figure was 40 per cent and 50 per cent respectively.
Writing in the British Medical Journal, the scientists emphasised that the risk was not seen in those on doses lower than 1mg, regardless of the duration of their treatment.
Dr Robert Storey, professor of cardiology at the University of Sheffield, who was not involved in the research, said the results demonstrated the importance of ensuring risk factors for cardiovascular disease were ‘well controlled’ in those taking oral HRT.
‘The study emphasises the superior safety of patches over oral treatment,’ he added.
Previous research has found that HRT – which can reduce hot flushes by 75 per cent – can prevent the brittle bone disease osteoporosis, while recent studies have suggested a link with the prevention of dementia.
