Magic mouthwash effective treatment for mouth sore pain caused by radiation therapy. 'Magic mouthwash,' an oral rinse containing diphenhydramine, lidocaine and antacids, significantly reduced pain from oral mucositis, mouth sores, in patients receiving radiation therapy for cancers of the head and neck when compared to plaecbo. Source 6u.
Taking mouthwash is kind of taking a risk at the same time. Yes it does kill the germs in your mouth but at the same time it can damage the gums or the good cells that are there as well. So according to the article, this situation also applies to radiation therapy.