It can also stain the heart and other tissues. But worth noting this was a much higher dose than your typical "biohacker" uses. The levels used in a hospital for things like methemoglobinemia, septic shock, whatever are many times larger.

Regardless, I don't understand why biohackers are often obsessed with things like methylene blue when targeted modulators usually exist. If someone is obsessed with "mitochondrial function" I'd expect him to look at methoxatin or something along those lines before something as broadly active as methylene blue. If someone wants MAO inhibition pharmaceutical options are probably better in terms of controllable isoform affinity. I don't see any reason to prefer something less well-studied for MAO inhibition over e.g. rasagiline or moclobemide.