A feasibility study, randomising to either 10 or 20 mg of tamoxifen. The primary aim is to identify the time to mammographic density change and piloting the procedures. It is known that a density change is seen after 12 months of 20 mg of tamoxifen, but the clinical experience indicates that an effect of drugs influencing mammographic density could be seen already after a few months. The second aim is to evaluate if there is a density change from 10 mg of tamoxifen equivalent to 20 mg.

The investigators want to conduct a feasibility study including 42 women who will be randomised to either 10 or 20 mg of daily oral tamoxifen. The primary aim of the study is to identify the time to mammographic density change and piloting the procedures. It is known that a density change is seen after 12 months of 20 mg of daily tamoxifen, but the clinical experience indicates that an effect of drugs influencing mammographic density could be seen already after a few months. The second aim of the pilot study is to see if there is a density change from 10 mg of tamoxifen equivalent to 20 mg and if there is a difference in adverse reactions between the two doses. Interim analysis will be performed at 6 months, monitoring both safety and level of mammographic density change.

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