Formaldehyde

Formaldehyde is a known irritant poison, capable of causing severe internal damage, and formic acid is equally destructive, best known to scientists as an essential part of the venom in bee stings.  People poisoned by methyl alcohol would often seem to recover from that first bout of dizzy sickness, feel better while the alcohol was being metabolized, and then ten to thirty hours later by poisoned again by the breakdown products.

First, their vision would blur.  The optic nerve and retina are acutely vulnerable to formic acid salts.  The nerve, with its continual processing of images, runs in a high metabolic state, causing blood to circulate through it rapidly — which causes poison to be delivered there continuously.  Autopsies often revealed a startling atrophy of the optic nerve area, the surrounding tissue swollen, bloody, and spongy.  Methyl alcohol and its by-products caused similar damage in the parietal cortex, a region of the brain essential in processing vision.  It concentrated as well in the hardworking lungs — the breakdown of pulmonary tissue was what usually killed people.

That is from the new and consistently interesting The Poisoners' Handbook: Murder and the Birth of Forensic Medicine in Jazz Age New York, by Deborah Blum.  Although she has won a Pulitzer Prize, she remains an underrated author.