Sorry for my slow response. I have been on vacation. Honestly, I don't have much experience with D-Ribose. I found this article on pubmed. I always err on the side of caution but this is an interesting finding. There doesn't appear to be an overwhelming amount of research available so it is likely an area in which more research is needed to fully understand the benefits/risks. (Also, note this study is in mice, not humans)
http://www.ncbi.nlm.nih.gov/pubmed/23438786