A review of 11 human studies on hot flashes2 concluded that it was the genistein that is responsible for the reduction in hot flashes and not the other isoflavones found in soy.
In another high quality clinical study with pure genistein employing 389 postmenopausal women3 the average number of hot flashes was significantly decreased at 1, 3, 6 and 12 months with the average percent reduction reaching 56% by the 12-month point. No further reduction in hot flashes occurred after 12 months but the benefit was maintained.
References
Elliott J et. al. Genistein reduces the number and duration of hot flashes in peri-post menopausal women in a randomized, double-blind, placebo-controlled, multi-center trial. Experimental Biology 2009 LB471.
Williamson-Hughes PS, Flickinger BD, Messina MJ, Empie MW. Isoflavone supplements containing predominantly genistein reduce hot flash symptoms: a critical review of published studies. Menopause 2006;13:831-839.
D’Anna R, Cannata ML, Marini H, Atteritano M, Cancellieri F, Corrado F, Triolo O, Rizzo P, Russo S, Gaudio A, Frisina N, Bitto A, Polito F, Minutoli L, Altavilla D, Adamo EB, Squadrito F. Effects of the phytoestrogen genistein on hot flushes, endometrium, and vaginal epithelium in postmenopausal women: a 2-year randomized, double-blind, placebo-controlled study. Menopause 2009;16:301-306.