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A new, large study has confirmed that different types of hormone replacement therapy (HRT) are associated with an increased risk for breast cancer and has provided additional information on factors associated with that increased risk.

The study also suggests that the risk may be lower than was estimated in a large meta-analysis of 24 trials that was published in 2019 in The Lancet. In that study, researchers suggested that the risk for breast cancer with HRT was higher and persisted longer than had been thought.

This conclusion from the meta-analysis was widely reported in the lay press and led to the UK Medicine and Healthcare Products Regulatory Agency issuing a safety alert for HRT regarding breast cancer. Experts in the field questioned the alert and said it caused undue anxiety. The European Medicines Agency also issued a safety alert because of the study.

This new study was begun before publication of the meta-analysis. Although the results are broadly similar in suggesting increased risk for breast cancer with HRT use, findings from the new study suggest that the risk is lower than had been estimated in the meta-analysis and that the risk diminishes more rapidly after stopping HRT than was suggested by the meta-analysis.

Because this study used a "consistent design" and "consistent data sources," these new results "are likely to be more accurate and reliable for assessing risks among HRT users.