Karolinska Interventional Study of Mammograhic Density (Karisma-1)

Inclusion Criteria: * Previously participated in the Karma study * Attending the national mammography screening program, i.e. aged 40-74 * A mammogram, including a stored raw image, must have been taken within 3 weeks of baseline and start of therapy * Having a mammographic volumetric density above the lowest 1/6, measured by Volpara * Informed consent must be signed before any study specific assessments have been performed Exclusion Criteria: * Being pregnant or planning to become pregnant during the study * Any previous or current diagnosis of breast cancer (including carcinoma in situ) * Recalled (mammographic code 3 or above) after baseline screening mammography * Any previous diagnosis of cancer, with the exception of non-melanoma skin cancer and in situ cancer of the cervix * Currently using oestrogen and progesterone-based hormone replacement therapy * Current use of hormone contraceptive with hormones, e.g. hormonal contraceptive pills, or progesterone implants. Hormonal intrauterine devices are accepted. * A history of thromboembolic disease such as embolism, deep vein thrombosis, stroke, TIA or cardiac arrest. * A history of immobilization, e.g. using wheelchair * Uncontrolled diabetes defined as known untreated diabetes * Hypertension at baseline, defined as systolic pressure higher than 140 mm Hg and diastolic higher than 90 mm Hg * Use of drugs that interfere with CYP2D6 expression such as paroxetine, fluoxetine and bupropion