system
Given the following post, respond as if you're commenting on Reddit:
human
Why does drinking methanol (CH3OH) cause blindness while drinking ethanol (CH3CH2OH) doesn't?

Even though the difference between the two is only one carbon.
gpt
The compound causing the blindness is actually not methanol, but formic acid, created from methanol in a metabolic process called toxication. Formic acid is a nerve toxin that acts through damaging mitochondria, and the optic nerve seems to be particularly vulnerable to its toxicity. Though, with doses only a little above the dose that can lead to blindness (that is, about 15 ml), toxicity extends to the whole organism with potentially lethal outcome. 

Fun fact: the antidote to methanol poisoning actually is ethanol. Poisoned individuals are being kept at about 1 per mille, for about a week, by means of infusion.