Formic acid or formate, a product of the acid, inhibits the action of an enzyme called cytochrome oxidase, which is vital to a cell’s ability to use oxygen. Under normal conditions, the eyes, specifically the optic nerve, use a huge amount of oxygen—that’s why a couple of the first signs of suffocation are tunnel vision and the loss of the ability to see color. So with a big enough dose of methanol, the eyes go first. And in fact people killed by methanol show a characteristic pattern of lesions on the optic nerve and in the brain.
