You don't need to fast while doing the P&B Colon Cleanse and it is not necessary to spend $250 or to take herbs to keep you going... in fact, those herbs are more than likely laxatives and not advised.
I like Southern Belles idea about taking your pills at night...just make sure that it is at least an hour before or after you take a shake. Also, keep in mind that any cleanse can result in changes to your hormonal system and menstruation patterns. Chances are there will be no problem...but you might want to use a back-up method to be extra safe when you are cleansing.