What makes someone a credible witness? I have 3 witnesses against me and all of them have been caught, and used drugs. I have not been caught with any drugs. This is a federal drug conspiracy case and I was wondering if they could be credible witnesses or not. I only know 1 of the witnesses but, the others say that this one guy has told them I was this person. Keep in mind that all are users of drugs and are all connected to this same conspiracy in some way or fashion.