Methylene Blue

Methylene blue is an old, genuinely approved medicine — a blue dye that's also a versatile redox-active drug. It has real clinical uses, most notably as the antidote for methemoglobinemia.

It can shuttle electrons inside cells, which at low doses lets it support mitochondrial energy production by bypassing certain blocked steps; clinically, it converts dysfunctional methemoglobin back into oxygen-carrying hemoglobin. Its effects are biphasic — helpful at low doses, potentially harmful (pro-oxidant) at high ones — and it's also a potent inhibitor of the enzyme MAO.

That MAO-inhibition is the key safety point: combining methylene blue with serotonin-raising drugs (like many antidepressants) can trigger dangerous serotonin syndrome. Its approved uses and dosing are well defined; the popular low-dose 'nootropic' use is far less established.