Oral HRT Linked to Increased Risk of Deadly Blood Clots, Study Warns

Taking hormone replacement therapy (HRT) pills for less than a year can increase a woman's risk of developing potentially fatal blood clots by 60 percent, according to a major study published in the British Medical Journal. The findings highlight a significant risk associated with oral HRT, particularly concerning venous thromboembolism, which is a condition where blood clots form in deep veins and can travel to the lungs.

Researchers analyzed health registry data from over one million Danish women aged 50 to 69 between 2003 and 2021. The study identified 9,807 cases of serious blood clots, 18,460 strokes, and 11,974 heart attacks. Each woman who experienced one of these conditions was matched with five women of the same age who had not, with statistical adjustments made for factors like medical history that could influence clot risk.

The analysis found that oral HRT, encompassing both estrogen-only and combined estrogen-progestogen treatments, was associated with a 60 percent higher rate of blood clots. This increased risk was observed even for those taking daily doses below 1mg, which is defined as low by the study authors, and regardless of the duration of treatment.

For women using HRT for more than five years at high doses (above 1mg daily), the risk of heart attack and stroke increased by 80 percent. For high-dose treatment exceeding one year, the risk for heart attack and stroke was 40 percent and 50 percent higher, respectively.

These risks are linked to how oral HRT is processed by the liver, which can increase the production of blood-clotting proteins. In contrast, transdermal treatments, such as skin patches, gels, and sprays, bypass the liver and do not appear to carry the same danger of blood clots.

While previous studies have linked HRT to an increased risk of blood clots, stroke, and heart attacks, this research provides a more specific understanding of how dosage and duration impact these risks. The study's authors emphasized the importance of selecting HRT types, doses, and durations carefully for patients vulnerable to clots.

Experts not involved in the research noted the study's findings underscore the importance of managing cardiovascular risk factors in individuals undergoing oral HRT. They also highlighted the superior safety profile of transdermal patches compared to oral treatments.