I think I may be having a bad reaction to mercaptopurine. Ever since I left the hospital I've had the most painful, thick, kinda green stool. I've spent 2 days trying to figure out and solve my problem but nothing works.. so I started to think, what has changed since I got out of the hospital that might be causing this.. the only thing that's changed was a new medication. Mercaptopurine. After a little googling I found out that an unlikely but serious side effect of this medication is black and tar like stools also called melena (wikipedia it) Has anyone else had this side effect from this medication?