Co-Codamol addiction causing live problems?

Hey..i am a 23 year old male and I am addicted to co-codamol..It started wen i was 16 when i had a slip disc, which caused siatica, and they seemed to be the only pain killers that helped..I am still on them now, but sometimes I would exceed the daily dosage.  What concerns me now is my liver, because i think it is damaged. The codiene in the tablets seems to be the only thing keeping my stool together, as once I come off them my stool comes out in a yellow liquid form, sometimes a dark black liquid. I recently went to my doctor and told him about my stool, but was too scared to tell him about the co-codamol addiction.  He got back to me with my blood tests and says i have a stomach virus, but surely i cannot have had this for nearly a year? He says my liver is fine, which i find hard to believe saying the amount of co-codamol i take.  If they wernt actually looking for liver problems would they have still have shown in the blood tests? I am on 3 different types of anti-biotics for this stomach virus, but I am truely convinced it is my liver with the problem..The main reason i take co-codamol now adays isnt for my back pain.. but to keep my stool in one piece!