Adverse effects can be sleepiness and dry mouth. But neither tends to be overwhelming. It can lower your blood pressure, so if you take it you need to have that monitored. But it can work well for hot flashes. Several studies have shown that soy supplementation will reduce hot flashes.
Soy contains phytoestrogens which can work like estrogens in the body.
Similarly, studies have shown relief from other phytoestrogens such as flaxseed and red clover. From German data, we know that the herb black cohosh can be helpful for some women. Data on other substances, such as ginseng, is more controversial. Unfortunately, traditional physicians in the U.S. receive very little education on herbal approaches. If a woman is interested in this approach, I usually refer her to a board-certified naturopath.