How does methanol become toxic after it enters the body?

Methanol becomes toxic to the body through the process of biological oxidation. While the body typically attempts to detoxify alcohols by oxidizing them using the liver enzyme alcohol dehydrogenase (ADH), this strategy is ineffective for methanol.1

When ingested, ADH catalyzes the oxidation of methanol into formaldehyde, which is subsequently oxidized into formic acid (as the formate ion). Both of these metabolic products are significantly more toxic than methanol itself, leading to severe health consequences such as blindness or death.12

Because the same enzyme, ADH, is responsible for processing both ethanol and methanol, medical treatment for methanol poisoning often involves the administration of diluted ethanol. This acts as a form of competitive inhibition, where the enzyme is kept occupied by the ethanol, allowing the kidneys time to excrete the methanol before it can be converted into its more dangerous metabolites.1