Originally Answered: can evening primrose oil help a underweight person w/ irregular period get a more regular period?
It definitely *can* help, but it depends on the reason for the irregular cycle in the first place. If you're underweight, that could be the reason in itself (although so many people have irregular periods now that it could be only one factor among many). Try to get your weight up a bit -- focus on getting enough protein and good fats (EFA's: omega oils from fish, flax seeds/oil, hemp seeds/oil, evening primrose, etc.). And then... there could be a lot of different supplements that would help, in addition to evening primrose (which, however, is probably a good start). St. Francis Herb Farm has a good herbal tincture (Vitex) with a typical mix of herbs: vitex (agnus castus), damiana, dong quai, perhaps licorice... I forget which things are in it for sure. It's advertised as a menopause remedy, but it's not just for that -- it helps balance female hormones in general. I've used it for an irregular cycle myself.
However, as someone has already suggested, you should get some more personalized help, from a doctor or (preferably) naturopath who can address your own specific needs, based on the history of your problem. These irregularities can have many different causes, and the remedy would need to be different in each case.
I would definitely advise you to get off the pill as soon as possible, though; of course it can be necessary in extreme cases (if you get hospitalized because of blood loss, for example, then by all means do whatever will keep you out of the hospital), but the pill isn't a cure: it only takes care of the symptom, not the cause, and artificially injects you full of synthetic or horse-derived hormones... it's the opposite of balancing your system -- it only screws it up more, and has side effects into the bargain. So do seek a natural remedy if you can; A natural path may take a bit more work than just taking the pill, but I think in the end you'll be glad you put in the extra effort.