I took Vitex to get my cycle back on track after dd's birth, and I really saw a difference in PMS symptoms, too. It is a hormonal regulator that acts on the Pituitary gland, basically getting it to do its job, as opposed to putting alien hormones into your body. (hee hee, alien hormones) However, it works best in synergy with something else, that something else would depend on you in particular. I took it mixed with Motherwort, which was my herbalist-friend's suggestion. Motherwort is a great nervine, I always use it for PMS, but I tend to have sluggish menses and this herb does stimulate my body, ie. It tends to give me cramps. That's good for me, better than bleeding 1 tablespoon a day for a week and a half....
The other herb she was considering was Mugwort (or Crone's wort) so if you're looking things up you can look that one up too.
I am thinking that Herb Pharm makes a vitex formula specifically for PMS, that has a very very small amount of Pulsatilla in it. Maybe you can find that.
Oh, if you are taking vitex to regulate your cycle (as opposed to the pulsatilla blend that would be just during PMS times) you need to take 1 tsp. of the blended tincture twice a day for at least 3 months, maybe longer, but it really does work if you stick with it....
edited to add: I have no idea whether pulsatilla is OK when you are nursing. Check into that first......
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