Board Thread:Naruto Discussions/@comment-24373005-20140811055011/@comment-4271790-20140812070747

Umishiru wrote: I think it is due to Obito being implanted with Hashirama's cells before he gained MS. They counter balanced the negative effects of the MS. Thus Kamui spamming. I used to think that as well. But now, I don't think Hashirama's cells had anything to do with the longevity of the Mangekyo Sharingan. Like @Hagoromo 1234 said, Itachi's Mangekyo Sharingan lasted for years, and he didn't have the first Hokage's cells. Hence the reason in the trivia sections of Kamui's and the Mangekyo Sharingan's articles, Obito's eye not deteriorating from regular use of Kamui are for reasons unknown.