It's been years since I did this research and can't find my listing of sources.

After a quick deep dive on the topic, I found this: http://www.ncbi.nlm.nih.gov/pubmed/18852035 In short, cells from oral tissues experienced relatively more DNA damage. In absolute terms, it's not much, but reason enough to default to patches, versus oral routes, IMO.

Also, oral nicotine causes all kinds of gingival recession. This is pretty widely known (halfway decent source for this: http://www.ncbi.nlm.nih.gov/pubmed/2388141).