Hey all, I found this info regarding vitex/chaste tree online:

YourHealth Australia - Integrative Medicine - Conventional, Natural, Alternative
Vitex does not contain hormones itself, however it has a unique ability to affect the regulation of human hormones via actions upon the pituitary gland. It has been shown to regulate the production and release of luteinizing hormone and prolactin in particular, which in turn affects ovarian follicle function and increases progesterone production. Vitex is perhaps the most widely used herb for female reproductive problems.
Conditions Commonly Used For
PMS
Irregular menstrual cycles, amenorrhoea
Acne
Heavy periods
Fibrocystic breast disease, lumpy breasts
Infertility
Progesterone deficiency
Peri-menopause
Elevated prolactin levels
Numerous female reproductive complaints, according to diagnosis

Side effects of using vitex are rare. Minor gastrointestinal upset and a mild skin rash with itching have been reported in less than 2% of the women monitored while taking vitex.
Vitex is not recommended for use during pregnancy or breastfeeding and should not be used at the same time as hormone therapy (eg. HRT, Oral Contraceptive Pill)