But mercaptopurine (6MP) is a different drug to Imuran (azathioprine). They're almost identical, but Imuran gets broken down into 6MP in the body. No idea what difference this extra step makes, but it must make some difference to make it worthwhile having the two separate drugs, instead of just 6MP alone.
I'm on 9mg of Entocort at the moment, due to go down to 6mg at the end of this month. On 50mg of 6MP, will go up to 75mg when I finish this current bottle (about 3 weeks). The Entocort doesn't seem to have had much effect, really... =/