Is 800 mg of advil (ibuprofen) at a time to much? and which is the better choice to slow down period flo? advil (ibuprofen) or apple cider vinager?

No divide the dose: It is good choice of treatment but should be in divided dose and take it after meal since it affect stomach resulting abdominal pain heartburn and sometime it causes stomach bleeding.

Yes: Are your symptoms that severe that you need to take so much advil? That's a very high dose. Advil can thin out your blood, so it may exacerbate your bleeding. I would recommend taking 200-400 mg at the most. If ur symptoms are that severe, then i would alternate it with some tylenol (acetaminophen). If you are regular, start taking the advil 1-2 days prior to your period. You can also try ssris to help your sympt.