I have read probably 30 book on migraines (and headaches in general) and have not heard of any significant side effects from Feverfew (everything you take has the possibility of side effects but I haven't seen anything other than diarrhea)
I personally have not taken Feverfew long enough to see results. I am just getting started on MigraHealth, a product that contains Feverfew, Magnesium and B Vitamin.
I am glad to hear that you have seen some type of reduction on headaches. I have waited until now to try this, because I have been on and off different things and wanted to know what specifically was or was not working.
Good luck with that...keep us posted!