Do phyto-oestrogens in soy reduce menopausal symptoms?
Soy isoflavones may reduce hot flushes somewhat, but the effect is modest and the studies contradict each other enough that they should not be relied upon blindly. They are a reasonable option if you want to avoid hormone therapy, but do not expect a miracle cure.
Soy isoflavones reduce hot flushes by an average of around 20 to 26 percent compared with placebo, and in absolute terms that amounts to roughly one fewer hot flush per day. Doses above 18.8 mg of genistein per day proved more than twice as effective as lower doses. A median of 54 mg per day was used in most trials. The results of the individual studies vary considerably, however, and a recent 2025 analysis found no significant effect on hot flushes at all. That contradiction has not been adequately explained.
Night sweats demonstrably respond less well. Multiple analyses found no significant effect for that symptom. For vaginal dryness there is a small, measurable benefit, but whether that is noticeable in practice remains unclear. And when you look at menopausal symptoms as a whole, the effect is disappointing: researchers found no significant improvement on a composite symptom score.
Preliminary signals from smaller studies suggest that soy isoflavones may also relieve headaches, palpitations and low mood, but the researchers themselves caution that those groups are too small for firm conclusions.
The available studies are reassuring on safety: no significant difference in side effects was found compared with placebo. There is also no indication of an increased risk of breast or uterine cancer, and at high dietary intake a possible protective effect is even observed, though that is currently too poorly studied to base anything on. The safety data cover only shorter to medium-term periods of use; what happens with long-term use has not been adequately investigated.
This answer draws on multiple analyses summarising dozens of randomised studies, including a large analysis in JAMA of 62 studies involving more than 6,600 women. Even so, the evidence is not clear-cut: the studies differed considerably in design, dose and outcome measure, 74 percent of the trials had a high risk of bias, and a more recent 2025 analysis found no effect on hot flushes. The researchers behind the JAMA analysis rated most of the evidence as of limited quality. That makes the positive results plausible, but not sufficiently proven.