Agnus Castus contains a few different constituents, including flavonoids, iridoid glycosides, and terpenoids. The whole fruit extract, rather than one of its individual constituents, appears to be necessary for the medicinal activity of agnus castus.¹ Agnus Castus does not contain hormones. The benefits of agnus castus stem from its actions upon the pituitary gland—specifically on the production of a hormone called luteinizing hormone (LH). This indirectly increases progesterone production and helps regulate the menstrual cycle. Agnus Castus also keeps prolactin secretion in check.^{2, 3} The ability to decrease mildly elevated prolactin levels may benefit some infertile women as well as some women with breast tenderness associated with premenstrual syndrome (PMS).

A controlled clinical trial found that women taking 20 mg per day of a concentrated agnus castus extract for three menstrual cycles had a significant reduction in symptoms of PMS, including irritability, mood swings, headache, and breast tenderness.⁴ Another double-blind trial found that women taking agnus castus had slightly greater relief from symptoms of PMS, including breast tenderness, cramping, and headaches, than those taking vitamin B6.⁵ These trials support the findings of preliminary agnus castus trials for women with PMS.^{6, 7} Agnus Castus (32.4 mg per day), in combination with some homoeopathic remedies, has also been found in a double-blind trial to successfully treat breast tenderness (also called mastalgia).⁸

A review of other trials and case reports suggests there is at least preliminary support that agnus castus should be considered for women with irregular periods, infertility, and mildly elevated prolactin levels.⁹ Double-blind trials have confirmed the effectiveness of agnus castus at lowering mildly elevated prolactin levels in women.¹⁰ According to one small trial, acne associated with PMS, may also be reduced using agnus castus.¹¹