Agnuside is an iridoid glycoside found primarily in the fruit of Vitex agnus-castus (chaste tree). This plant extract has a longstanding traditional use for supporting menstrual health, particularly premenstrual syndrome (PMS). PMS Type S refers to the "Syndrome" or "Sugar" subtype, characterized by symptoms such as irritability, mood swings, and craving for sweets, often associated with hormonal imbalances, especially elevated prolactin or altered dopamine activity.

Scientific studies have validated the efficacy of Vitex agnus-castus extracts in reducing PMS symptoms. Agnuside is considered one of the principal active components responsible for the dopaminergic effects of the extract, which lead to decreased prolactin release from the pituitary gland. Elevated prolactin is implicated in various PMS symptoms, and its reduction can help re-balance hormonal cycles. Multiple randomized, placebo-controlled trials (e.g., Schellenberg, 2001; Loch, 2000) have shown that standardized Vitex extracts significantly improve overall PMS symptoms, including irritability and mood swings characteristic of PMS Type S, compared to placebo.

However, most clinical trials have studied the whole extract rather than isolated agnuside. Thus, while the presence of agnuside is considered essential for the extract's activity, direct clinical evidence for agnuside alone is limited. The evidence for Vitex agnus-castus in PMS is strong (meta-analyses rate it as moderate to high); the evidence specifically for isolated agnuside, while mechanistically plausible, is less robust. Therefore, the rating given reflects the strength of evidence for the extract as a whole, with agnuside as a likely active constituent.