I agonized too, about it. In fact, I bought some, then didn't use it for a few months.
This is my second month. I must tell you that I wasn't going to use it again, but found myself slipping into that cranky attitude, so I kept it up (good for one's marriage;)
I use Products of Nature, which recommends 1/4 tsp 2 x/day starting on day 12 (first day of your period being day 1) and stopping on day 26 (if you're still menstrating for first two month, then reduce to 1/8 tsp. 2x/day for same time frame.
I found that 1/4 tsp. seemed like a bit much for me (my breasts got sore). I cut back to the 1/8 dose on month two.
I hope this helps.
Good luck,
KarenMarie
p.s. My friend just started using it too, and she *swears* it helps her mood big time.