If the pilot study is successful, the second part of the study will be done. This 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 it in pill form may reduce the amount of drug that enters the blood stream and cause less side effects.