I've been taking them since week one and am now down over 60lbs so no problems there.
Start with two a day and see if you get a bit more regular. Wait a few days before you up the dosage. Remember to drink an extra 250mls of water with them. These work by drawing water from you body into the bowel and then swell up into a glutinous mass that helps your bowels move.
Also remember that you will only "go" every couple of days as your body has no solids to digest.