My girlfriend takes an advil a day, how bad can this be?

My girlfriend just told me that she gets extreme headaches but usually she can maintain them by taking an advil a day. She went to the doctors and they couldn't find anything wrong with her. She also takes birth control but I have no idea if that could be the cause, Also I don't know if she got the headaches before taking the pill. I know advil can effect your kidneys and intestines but how severe is this really?