Phytoestrogens are plant-derived compounds with structural similarity to human estrogen. They are found in foods such as soy, flaxseed, and certain herbs. Their use in supporting or treating menstrual irregularity is based on their ability to weakly bind to estrogen receptors and exert estrogenic or anti-estrogenic effects depending on endogenous hormone levels.

Several clinical and observational studies have evaluated the effects of phytoestrogens, particularly isoflavones from soy, on menstrual cycle characteristics. Some evidence suggests that phytoestrogens can influence menstrual cycle length and hormonal profiles in premenopausal women. For example, a few small randomized controlled trials (RCTs) and cohort studies have reported slight increases in cycle length or alterations in hormone levels with high phytoestrogen intake. However, the results are inconsistent, and many studies have methodological limitations, such as small sample sizes and short durations.

Overall, the scientific evidence supporting the use of phytoestrogens for menstrual irregularity is limited and not robust. While there is a plausible biological mechanism and some preliminary data, high-quality, large-scale RCTs are lacking. Authoritative reviews, such as those by the Cochrane Collaboration and systematic reviews, generally conclude that more research is needed before phytoestrogens can be recommended for this indication. Thus, the evidence is rated as 2 out of 5.