The blue dye methylene blue cut the amount of a harmful chemical (hydrogen peroxide) in heart and blood vessel fat by about half when tested in the lab.

When heart and blood vessel fat from heart failure patients was treated with methylene blue, the amount of harmful reactive molecules (like hydrogen peroxide) it produced went down.

The more methylene blue you add, the more hydrogen peroxide the mitochondria make—up to 25 times more at the highest dose tested.

Methylene blue makes mitochondria produce more hydrogen peroxide, but if you block a specific part of the energy chain (Complex III), that extra production stops—meaning methylene blue needs that part to work.

Two chemicals, methylene blue and azure I, make mouse brain cells produce a lot more hydrogen peroxide — over two and a half times more in some cases — when tested in a lab dish. This suggests they cause the tiny energy factories inside the cells to make more of this potentially harmful substance.

Methylene blue makes heart cell power plants produce more harmful molecules when using one fuel (glutamate/malate), but less when using another (succinate), showing its effect depends on what the cell is burning.