Researchers from Iran note that among a group of women they randomly assigned to take either St. John's wort or a placebo for eight weeks, those taking the herb had a greater reduction in daily hot flashes. Among women taking the herb, the average number of hot flashes was lowered  from roughly four per day to two per day after several weeks of use. The herb also reduced the duration and severity of the women's hot flashes. The study included 100 women who were 50 years old, on average, and had been experiencing moderate to severe hot flashes at least once per day. See http://www.raysahelian.com/stjohn.html