Aaargh again!

My IBS pills have stopped working. My first lot, the ones that worked, came in a white packet with a bit of yellow on. I now have two new lots, one in a green and white packet and one in a purple and white packet. I tried the greenies for a couple of days and felt awful. Then I took the purplies and still feel awful. I had this problem with my ulcer tablets. Ranitidine in a white packet worked fine, but in a green packet didn't work at all. My god-daughter has found the same thing. Even Mr G has had this problem. Does every factory that makes them, use a slightly different method? There must be some small variation, but what? My stomach is right back to where it was before I started taking Mebeverine. I woke at 6 am with awful stomach pain, have made several dashes to the loo, feel no better and have to face a trek to the dentist of an hour and a half each way on public transport. Just not fair!