It's on my short list of things to read more about/think about.
It's antifungal, which could be a good thing for those needing it, but also can bring side effect symptoms if the yeast/fungi being killed are killed quickly. (die-off / herxheimer type symptoms )
A fairly small amount is absorbed from the gut, about 1/15th of the dose, according to this:
http://www.ncbi.nlm.nih.gov/pubmed/10952480
So the purported memory-enhancing and mitochondrial improving effects of the stuff, if true, are from the small amount that does make it from the gut into the blood.
The rest of it, that works its way down the gut and out of the body, would kill some yeast/fungi in the gut...in theory.
I say all this because this could explain why some people get awful headaches from it, and others don't, from the same dose.
Longecity.org has some discussion on it, but if you aren't a member of that site, the only way to read them is to do a google search from outside the site: methylene blue site:longecity.org in your google or whatever browser you use.
A quote from some of those discussions:
(from someone who got a nasty headache from it)
"my sister has been using the same source of MB as me (with the part intention of helping a semi-perm staph infection go permanently away) and she experienced
none of the above symptoms that plagued my experience with MB"
The thread I found that is here:
http://www.longecity.org/forum/topic/51983-methylene-blue-experiences/