If this pilot study is successful, then the second part of the study will be conducted. The second part of this study will test the effects of a 0.1% (20mg) thalidomide mouthwash in treating oral cGVHD-related stomatitis in patients following allogeneic HSCT. Applying thalidomide directly to the GVHD-related mouth ulcer in gel form or to the entire oral cavity in mouthwash form rather than taking the thalidomide in pill form may reduce the amount of drug that enters the blood stream and cause fewer side effects.