Common side effects include nausea, infections, and diarrhea.[1] Other serious side effects include an increased risk of cancer, progressive multifocal leukoencephalopathy, anemia, and gastrointestinal bleeding.[4] Use during pregnancy may harm the baby.[4] It works by blocking inosine monophosphate dehydrogenase (IMPDH), which is needed by lymphocytes to make guanosine.[4]