Methylene blue
Also called: methylthioninium chloride.
Last reviewed 2026-09-16. How this works.
When to call your team
Contact your care team or seek urgent medical advice the same day if you notice:
- feeling confused, agitated or unlike yourself
- shivering, sweating or a racing heart
- muscles twitching, jerking or becoming stiff
- diarrhoea or being sick alongside any of the above
- yellowing of the eyes or skin, or urine turning very dark
The first four can be signs of serotonin toxicity, which has been reported with this class of product and has been fatal in some reported cases, most often when taken alongside antidepressants. The last can follow the breakdown of red blood cells in people with a common inherited enzyme deficiency. These are reasons to seek advice. They are not a diagnosis, and this page cannot tell you what is causing them.12
These are reasons to make contact, not a diagnosis. Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it.
This block is about what to watch for, and it is not all this page holds: an interaction recorded against a treatment class and what has been recorded about harm are below. Nothing in this block is an all-clear for a question it does not mention.
Treating cancer Grade D
No human evidence of benefit, or none that survived testing.
Harm has been recorded in people for this claim. The grade measures how strong the evidence for benefit is; it says nothing about the harm, and the two are read from different parts of this page.
No trial has tested this in people with cancer, and mixing it with some antidepressants has killed people.
No human trial, and a documented interaction that has been fatal.
The harm
Methylene blue is a potent reversible inhibitor of monoamine oxidase A. Product labelling warns that serotonin syndrome has been reported with this class of product, most often alongside serotonergic drugs including the commonly prescribed antidepressants, and that some reported cases were fatal. Antidepressants are prescribed very frequently to people with cancer, which is what makes this more than a footnote.1
It is also contraindicated in glucose-6-phosphate dehydrogenase deficiency, an inherited condition concentrated in populations of Mediterranean, African, Middle Eastern and South Asian ancestry, because it can cause red blood cells to break down.2
It inhibits liver enzymes that clear a number of other drugs, which can allow those drugs to build up. The register does not list which, and does not check drug against drug. That is a question for a pharmacist with your full medication list in front of them.2
The claim
The claim is that methylene blue, an old dye with medical uses, acts against cancer. It has become one of the more visible compounds shared online.
What it does
Proposals exist involving cell energy production. None is what this entry turns on.
The evidence
No trial in people with cancer is recorded in this register. What is recorded instead is the interaction profile, because that is the part a reader considering this needs.
What is absent
No trial of methylene blue in people with cancer is recorded in this register.
Scope: any tumour type, any endpoint.
Nobody has run a search to establish this, so it is the register’s reading of what exists rather than a gap somebody has looked for and confirmed.
What to ask
Would methylene blue interact with any of the other medicines I take, including the ones for mood?
The dish-to-person gap
The register records no human pharmacokinetic measurement for methylene blue against any laboratory concentration used in cancer work.
Interactions
This compound has a documented and potentially fatal interaction with a widely prescribed class of antidepressants, and it inhibits liver enzymes that clear a number of other drugs. The register does not run a drug-level interaction check and will not do so. It records the harm, and the reason to put the question to a pharmacist.12
Methylene blue
Recorded as documented, and the source names no particular treatment.
Can you check methylene blue, which is recorded as interacting with medicines that act on serotonin and with some liver enzymes, against everything on my list?
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
A trial is the setting in which the interactions on this page would be checked before anything was given.
Search ClinicalTrials.gov for recruiting studies of methylene blue
A trial is the same decision with monitoring, a known preparation and a doctor who knows what else you are taking.
References
Product labelling for methylene blue class products, and a paper titled Methylene blue and serotonin toxicity: inhibition of monoamine oxidase A confirms a theoretical prediction.
doi:10.1038/sj.bjp.0707430 · PubMed 17721552
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 20 note 5.
Product information for methylene blue recording contraindication in glucose-6-phosphate dehydrogenase deficiency and a list of drugs that may accumulate through inhibition of cytochrome P450 isozymes.
No persistent identifier recorded.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 20 note 6.