How painful is a bone marrow transplant?

Variable. The actual transplant which is the reinfusion of the bone marrow or stem cells is not painful. However there can be side effects to the radiation and/or chemotherapy used to treat the patient before the stem cell infusion. Depending on how the transplant is done those treatments can cause some unpleasant side effects including nausea, vomiting, abdominal pain and mouth sores among other things.