Methylene blue: the MAOI interaction is not a footnote

Methylene blue is showing up in dropper bottles marketed for focus and brain fog, and I want the interaction stated loudly in a thread people can find.

It is a potent monoamine oxidase inhibitor. Combined with an SSRI, SNRI, tricyclic or other serotonergic drug it can cause serotonin syndrome. That is not a theoretical pharmacology-textbook interaction — it has caused deaths in hospital settings, in controlled conditions, with clinicians present.

SSRIs are among the most prescribed medicines there are. The overlap between "people who would buy a focus supplement" and "people on an antidepressant" is not small.

Seconding this as hard as I can.

What makes it particularly dangerous in this context is that the approved use is intravenous, in hospital, for methaemoglobinaemia, where the interaction is known and managed. The bottle sold online for focus is bought by somebody who has told no clinician, whose GP does not know, and who does not think of a dye as a drug.

The second safety note on that page is also real and less well known: G6PD deficiency. In people who have it, methylene blue can cause severe destruction of red blood cells. It is common in some populations and a large fraction of people who have it do not know they do.

So the honest summary of the compound: a genuinely excellent emergency medicine, on essential medicines lists, with a hundred and fifty years of history, and two specific ways of harming somebody who takes it casually.

And note the compound-page status is approved with evidence: mixed, and those two labels are about different things.

Approved for methaemoglobinaemia, where it works quickly and reliably. Mixed evidence for what people are actually buying it for — focus, mood, mitochondrial function, long COVID — which is a completely different set of claims and was not what any approval assessed.

"It is an approved medicine" is true and is being used to import confidence into indications the approval says nothing about.

The dose gap is worth naming too. The hospital use is IV at a defined dose in a monitored setting. The dropper is oral at much lower concentration, self-administered, and the "much lower concentration" is often deployed as though it dissolved the interaction risk.

It does not. MAO inhibition is not a threshold that low doses politely stay under.

That is the point I most wanted made. Thank you.

If anybody reading this takes an SSRI, an SNRI, a tricyclic, or anything else serotonergic, and is considering this: talk to a pharmacist. Not a forum. A pharmacist will check it in thirty seconds and it is free.

Leaving this one prominent. It is the clearest example on the site of an acute-harm fact — the kind that does not move whatever else gets rewritten.