Chastetree (Vitex agnus-castus), also known as vitex or monk’s pepper, has a long history of traditional use for menstrual irregularities, but modern scientific studies have provided some validation for its use in this context. Research suggests that extracts of chastetree fruit may help regulate the menstrual cycle and alleviate symptoms of premenstrual syndrome (PMS) and other menstrual disorders, likely due to its effects on the pituitary gland and modulation of prolactin levels. Several randomized controlled trials have found that chastetree can be effective in improving cycle regularity and reducing symptoms associated with menstrual irregularities, such as luteal phase defects and mild hyperprolactinemia. Meta-analyses and systematic reviews generally indicate a moderate level of evidence supporting its efficacy, though studies often vary in quality and methodology. The active constituents, including flavonoids and diterpenes, are thought to interact with dopamine receptors, thereby influencing hormonal balance. While chastetree appears to be safe for most women, more robust, large-scale studies are needed to fully establish optimal dosing and long-term safety. Regulatory authorities in Europe, such as the German Commission E, recognize chastetree for menstrual disorders, reflecting both traditional and scientific endorsement. Overall, current scientific evidence moderately supports the use of chastetree for menstrual irregularity.