Short term HRT may raise blood clot risk – study

Short-term oral HRT was linked to a 60 per cent higher rate of serious blood clots in women aged 50 to 69, a large study found.

The increased rate was seen among women using oral hormone replacement therapy for less than a year as well as those treated for more than five years.

By contrast, transdermal HRT, delivered through skin patches, gels or sprays, was not associated with the same increased clot risk.

 

Researchers at North Zealand Hospital in Denmark analysed health registry data from more than one million Danish women aged 50 to 69 between 2003 and 2021.

During the study period, 9,807 women experienced venous thromboembolism, a serious blood clot in a vein. A further 18,460 had a stroke and 11,974 had a heart attack.

Each affected woman was matched by age with five women who had not experienced the same condition. Researchers adjusted the analysis for other factors, including medical history, that could influence risk.

They then examined HRT prescriptions, including the dose and duration of treatment, and looked for associations with blood clots, stroke and heart attack.

Overall, oral HRT was associated with a 60 per cent higher rate of venous thromboembolism, with increased rates seen for both oestrogen-only therapy and treatment combining oestrogen with progestogen.

Venous thromboembolism occurs when a blood clot forms in a vein. A clot in a deep vein can break away and travel to the lungs, causing a potentially fatal pulmonary embolism.

Women taking oral HRT for less than a year had the same raised blood clot risk as those using it for more than five years.

Even a daily oral dose below 1mg was found to significantly increase blood clot risk.

Women taking a daily oral dose above 1mg for more than five years had an 80 per cent higher risk of both heart attack and stroke.

For women taking a high oral dose for more than one year, the study found a 40 per cent higher risk of heart attack and a 50 per cent higher risk of stroke.

The researchers highlighted the importance of choosing the type, dose and duration of treatment carefully, particularly for patients vulnerable to blood clots.

Dr Robert Storey, professor of cardiology at the University of Sheffield, who was not involved in the research, said the findings demonstrated the importance of ensuring cardiovascular risk factors were “well controlled” in women taking oral HRT.

He added: “The study emphasises the superior safety of patches over oral treatment.”