This is a proof-of-concept phase 2 clinical trial to investigate the effect of the phytoestrogenic supplement PhytoSERM on vasomotor symptoms and other symptoms associated with the menopausal transition, and on blood-based biomarkers in peri- and postmenopausal women. After the screening period, participants will be randomized to PhytoSERM 50 mg pills (administered orally, once per day) or matching placebo, 1:1 allocation, for a period of 12 weeks. After 12 weeks, all participants in the placebo group will be crossed-over to receive PhytoSERM pills for the remainder of the study (open-label phase).