I've been on 50 mg per day of 6MP for about four years now. It has definitely reduced the inflammation (and hence the narrowing) in my terminal ileum. If it's producing side effects, I don't know about them, since most of them are the same as my Crohn's symptoms (nausea, diarrhea, lightheadedness). I have been totally reckless with my own health and safety, doing things in the toxic post-Katrina environment that should have killed me many times over. (I'm not kidding about this. You don't even want to know!) But I've only gotten sick once, with a respiratory virus. My doctor just says that I must have an amazing immune system, since it seems to be operating so effectively even with a daily drug to suppress it.
I have blood tests and liver function tests every three months. The 6MP has done nothing to relieve all my horrible Crohn's symptoms, but neither has anything else I've tried. Still, since the drug has helped to prevent any further obstructions since my surgery in 1999, I'm happy to be on it.
I've always heard 50mg per 50lbs. I am 115 and when I was on it it was 100mg. 150 sounds very high.