I have to agree with David, they are not the same drug, not similar. It is part of the 5 ASA drug class, similar to Asacol, Apriso, Mesalamine, Lialda, etc. 6MP is in a completely different drug class. Perhaps the doctors won't prescribe 6MP to you for a different reason.
In response to "Mugsy", I was on Cimzia and 6MP for quite some time. They put me on the 6MP when the Cimzia stopped working. The 6MP did nothing positive for me, although everyone is different. In fact, it dropped my WBC so low that every time I left my home, I would get a fever. I stopped both drugs and switched to Remicade. I've been in remission since doing so.