Many people are allergic to soy since it is used in so many food processing plants. Soy supplements may or may not cause allergies. If you are allergic but would like the benefits of reduced hot flashes, Physicians Committee for Responsible Medicine (PCRM) says “Women taking soy isoflavone supplements for six weeks to 12 months reduced the frequency of hot flashes by 21 percent, compared with women taking a placebo.”