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There are two points of view for the answer of this question:

  • The biological view, the only one that I faced during my research, states that since it can trigger perilous conditions like metabolic acidosis, cause a neurologic sequelae, etc. methanol is toxic.
  • The chemical point of view in an answer, which is the one I seek, is the reactions that occur in the body with methanol as a reactant.

Why should it be a reaction? Because I assume something will be toxic for humans when it reacts and disables some of their bodies' vital compounds; e.g.: Hydrogen cyanide reacts with enzymes, renders their active sites useless, and finally causes heavy reduction in bio-chemical reactions that results in quick death.

So, the question is: What reactions use methanol in the body that makes it toxic for us?

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    $\begingroup$ There's a third point of view wherein methanol is said to encourage harmful parasite behavior (allowing them to complete their life cycle without an additional host, and migrate to other organs, or some such). Parasites are said to excrete toxins, and the presence of toxins in more organs may cause issues. This isn't a direct conversion of methanol to toxins, however. So, I'm not sure how on-topic this thought is, but if true, it relates to your question. $\endgroup$
    – Brōtsyorfuzthrāx
    Dec 20, 2014 at 0:58
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    $\begingroup$ Related: This BioSE question $\endgroup$
    – Always Confused
    May 11, 2017 at 6:32

4 Answers 4

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Methanol isn't particularly toxic in and of itself, although it's no walk in the park.

If methanol flowed through the body without being broken down, it would cause roughly the same kind of harm as ethanol, i.e. intoxication.

The real culprit is one of its metabolic products, methanoic acid, also known as formic acid.

To understand how formic acid, present as the formate ion, is toxic, we look to Wikipedia:

Formate is toxic because it inhibits mitochondrial cytochrome c oxidase, causing the symptoms of hypoxia at the cellular level, and also causing metabolic acidosis, among a variety of other metabolic disturbances.

Edit: As DavePhD points out, an intermediate product in this process is formaldehyde, or methanal. While formaldehyde is also toxic, it is rapidly metabolized to methanoic acid.

Reedit: The deeper, more historical reason that this happens is that methanol isn't readily available in nature, meaning that few species have developed biochemical tools to deal with it. There simply hasn't existed an evolutionary pressure to deal with methanol.

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    $\begingroup$ Note that formic/methanoic acid is essentially ant poison. Imagine being stung by 100 ants... $\endgroup$
    – sadljkfhalskdjfh
    Mar 27, 2016 at 11:35
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The enzyme alcohol dehydroganase converts the methanol to formaldehyde in the body. Formaldehyde is then converted to formic acid.

Formaldehyde can cause blindness before being converted to formic acid, while formic acid causes acidosis as Williham Totland points out.

See Biochemical Aspects of Methanol Poisoning for more information.

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  • $\begingroup$ Note that the normal purpose of of alcohol dehydrogenase is to convert ethanol into acetaldehyde (which is also toxic, but there's a metabolic pathway to convert it into harmless acetic acid.) $\endgroup$
    – Mark
    Dec 20, 2014 at 0:46
  • $\begingroup$ @Mark yes, this is why ethanol has been given as a treatment for methanol poisoning. Ethanol competes with the methanol for binding to alcohol dehydrogenase, and the methanol can exist the body, for example through breathing, without being metabolized to toxic formaldehyde and formate. $\endgroup$
    – DavePhD
    Dec 20, 2014 at 12:38
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I just wanted to point out that everything is toxic in the right proportion. THe problem is with the dose. I think you could drink a bit of methanol without getting particulary harmed. Do not do it, obviously.

The LD50 is very low at 5g/kg.

Edit: Nothing to do with methanol, but it is a good infographic enter image description here

Source: http://www.compoundchem.com/2014/07/27/lethaldoses/

As a comparison with ethanol, I believe (not sure) that the oxidation product of these two compounds, ie, acetic acid and formic acid are not equally reactive. The former (metabolite of ethanol) is less reactive than the latter (metabolite of methanol). At least from an organic point of view this makes sense. But maybe there is another more biological reason.

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    $\begingroup$ There is a more biological reason: Ethanol presents in serious quantities in nature, available to animals, usually in the form of rotting fruit. Methanol is comparatively rare, so the body doesn't have any reasonable defences against its metabolites. $\endgroup$
    – Williham Totland
    Dec 19, 2014 at 22:13
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    $\begingroup$ @WillihamTotland Great apes are more tolerant of ethanol than most animals. $\endgroup$
    – 200_success
    Dec 19, 2014 at 22:25
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    $\begingroup$ Might be worth noting that even a sub-lethal dose of methanol (such as a small accidental gulp during 'Fire-eating' can result in permanent blindness. $\endgroup$
    – Dan Bryant
    Dec 20, 2014 at 2:18
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    $\begingroup$ 118 * 240 mL = 28 L >> 6 L...using "coffees" is maybe not that great as a unit. $\endgroup$
    – Nick T
    Dec 20, 2014 at 7:59
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    $\begingroup$ @AlteredState When an LD50 value is given, for example in an MSDS, it is almost always for an non-human animal. For humams "Ingestion of from 70 to 100ml of methanol is usually fatal" Biochemical Aspects of Methanol Poisioning, Biochemical Pharmacology, vol. 11, page 406. People have died from as little as 30ml, and blindness takes even less. People have gone totally blind from drinking only 5ml and even just acidentally spilling methanol on their skin. $\endgroup$
    – DavePhD
    Dec 22, 2014 at 16:30
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Old thread but...

Methanol toxicity is attributed to formic acid.

But formic acid itself is a "low-toxicity" food additive E 236 (authorized in animal feed only though, at ~1%, afaik), which is a bit confusing.

I assume formic acid derived from methanol/formaline metabolism has different fate-distribution-bioavailability than ingested formic acid. Or it's simply a matter of dosage.

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