Here's a snipit from the Adverse Reactions of Imuran:
Gastrointestinal: Nausea and vomiting may occur within the first few months of therapy with IMURAN, and occurred in approximately 12% of 676 rheumatoid arthritis patients. The frequency of gastric disturbance often can be reduced by administration of the drug in divided doses and/or after meals. However, in some patients, nausea and vomiting may be severe and may be accompanied by symptoms such as diarrhea, fever, malaise, and myalgias (see PRECAUTIONS). Vomiting with abdominal pain may occur rarely with a hypersensitivity pancreatitis.
Source:
http://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=aaa6c540-4c84-48a0-939c-cd423134fa2a#nlm34084-4