This literature review explores the breast cancer risk associated with combined hormone replacement therapy (HRT), which delivers oestrogen and progesterone. An increase in breast cancer risk was reported with all three routes of administration of progesterone, but the review also highlighted a difference in risk levels according to administration routes. It is suggested that future research controls for this variable when investigating the link between breast cancer risk and combined HRT. Making recommendations for which administration route of progesterone is associated with the lowest risk of breast cancer would be premature and inappropriate at this stage. There is a need for direct clinical comparisons to accurately and reliably create a hierarchy of administration routes and inform risk balancing and decision-making in menopause care and HRT prescribing.