It wouldnt be from the juice except as a side effct of your bodies cleansing .The pain is the fact that it is so cleansing. You can drink as much as you can stand, its pure nutrician. 8 oz a day is common for those really into it.How ones body responds reveals how congested we are. The smell makes many nauseaus.
I have always felt it most efficient to dilute it so as to assist the body in utilizing it. Taken in tonic form you feel it slowly permeate your system rather than where when taken straight and it can be quite nuclear.
I put 6 to 8 0z of wheat juice into a half gallon of water and add a guart or so of any juice without fiber. Taken in 8oz increments it is supreme.