High doses of methylene blue compete with oxygen by acting as a proton acceptor and oxidizing tissues.

“at a higher dose instead of acting as an electron cycler it actually replaces oxygen in the you know it competes with oxygen remember that I told you oxygen was the one that was taking the electrons so he can be you wanted to have it in a low concentration that is giving electrons and taking electrons are the same rate but you're saying at a higher dose it becomes a proton acceptor little yes because it competes with oxygen and it oxidizes the tissues and actually that was in part the first application that early found for methylene blue”