This article is general information, not medical advice. Methylene blue is a prescription medicine in many countries and can interact with common drugs. Talk to a doctor or pharmacist before taking it in any form.
Methylene blue is a dye that can easily gain and lose electrons. Inside the body that chemistry does a few things:
- It helps turn methemoglobin back into normal hemoglobin. This is its main approved medical use. It treats a blood condition called methemoglobinemia, in which red blood cells can’t carry oxygen properly.
- It inhibits an enzyme called monoamine oxidase A (MAO-A). That’s why it can interact dangerously with many antidepressants and some other drugs.
- It can act as an electron carrier in mitochondria. This is the basis of much of the research interest, and of many online claims, but the evidence for everyday health benefits is limited.
- It colours things blue or green, including urine, which is harmless but alarming if you aren’t expecting it.
In medical use it’s usually given as an injection, under supervision, at doses chosen by a clinician. The rest of this article explains each effect, the approved and experimental uses, and the safety issues.
What methylene blue is
Methylene blue (also called methylthioninium chloride) is a synthetic dye first made in the 1870s. It was one of the earliest synthetic compounds used as a medicine: in the 1890s, Paul Ehrlich and colleagues reported treating malaria with it. It’s still widely used as a biological stain. Microscopists use it to make nuclei and bacteria visible, as we describe in our guide to methylene blue staining for microscope slides. It’s also sold as an aquarium treatment and has industrial uses as a dye.
Methylene blue exists in two forms:
- The oxidised form, which is blue.
- The reduced form, leucomethylene blue, which is colourless.
It switches between the two easily, accepting and giving away electrons. Almost everything it does in the body comes from that ability.
How it works in the body
Methemoglobinemia
Hemoglobin carries oxygen using iron in a particular chemical state (ferrous iron). If that iron is oxidised to the ferric state, the molecule becomes methemoglobin, which can’t carry oxygen properly. Some drugs, chemicals and inherited conditions can push methemoglobin to dangerous levels.
Inside red blood cells, an enzyme system that uses NADPH converts methylene blue into leucomethylene blue. The leucomethylene blue then donates electrons to methemoglobin, turning it back into normal hemoglobin. This is why methylene blue is an approved treatment for acquired methemoglobinemia in the United States and elsewhere, given intravenously in a hospital.
One complication: at high doses, methylene blue can itself oxidise hemoglobin and cause methemoglobinemia. Dose matters a great deal, which is one reason it’s given under medical supervision.
MAO inhibition and serotonin
Methylene blue is a potent inhibitor of monoamine oxidase A, an enzyme that breaks down serotonin and other neurotransmitters. On its own that may affect mood, which is why it has been studied in psychiatry. Combined with drugs that raise serotonin, though, it can cause serotonin syndrome, a potentially life-threatening reaction. Those drugs include SSRIs, SNRIs, some other antidepressants and some pain medicines. In 2011 the US Food and Drug Administration issued a safety warning after reports of serious serotonin syndrome in people given methylene blue while taking serotonergic psychiatric medicines.
Mitochondria and oxidative stress
Methylene blue can shuttle electrons within the mitochondrial electron transport chain, partly bypassing some of its usual steps. In laboratory and animal studies this has been linked to changes in cellular energy production and oxidative stress. These findings are why researchers have explored it for conditions such as Alzheimer’s disease. They don’t by themselves show that taking it improves health in otherwise healthy people.
Nitric oxide and blood vessels
Methylene blue interferes with the nitric oxide signalling pathway that relaxes blood vessels. In hospitals it’s sometimes used off-label to treat vasoplegia, dangerously low blood pressure from widened blood vessels, for example after heart surgery. This is a specialist use in intensive care.
What it’s used for in medicine
Approved uses (these vary by country):
- Treatment of acquired methemoglobinemia, by intravenous injection.
Other established or off-label clinical uses include:
- A dye during surgery, to help surgeons see certain tissues, find leaks, or map lymph nodes and glands.
- Treatment of vasoplegic shock in some intensive care settings.
- Treatment or prevention of encephalopathy (brain toxicity) caused by the chemotherapy drug ifosfamide.
- As part of treatment in some poisonings.
- Research into pain relief, for example a methylene blue mouth rinse studied for mouth sores caused by cancer treatment.
Historical uses: malaria treatment, and as a mild urinary antiseptic, often in combination products.
The wellness and “nootropic” claims
In recent years methylene blue has been promoted online for focus, memory, energy, mood, anti-ageing and more. It’s sold as drops and capsules, sometimes without prescription.
What the evidence supports, cautiously:
- There’s genuine scientific interest in its effects on mitochondria and the brain. Some small human studies have looked at memory and brain activity.
- A modified form (LMTM, a stabilised version of leucomethylene blue) has been tested in Alzheimer’s disease trials, with mixed results.
What it doesn’t support:
- Good evidence that healthy people get meaningful, lasting benefits from taking it.
- Evidence that unregulated products contain what they claim, at a consistent dose and purity.
If you’re considering it for a health reason, speak to a clinician, especially if you take any regular medicines.
Is methylene blue toxic?
Like most drugs, methylene blue’s safety depends on the dose, the person and what else they’re taking. In appropriate medical doses it’s generally well tolerated by suitable patients, but it does have side effects and some serious risks.
Common, mostly harmless effects:
- Blue or green urine, and sometimes blue-green stools, for a while after a dose.
- Bluish discoloration of the skin or mouth.
- Nausea, headache, dizziness, or pain at an injection site.
Serious risks:
- Serotonin syndrome when combined with serotonergic drugs (see above). Symptoms can include agitation, confusion, fever, sweating, muscle twitching and a fast heart rate. It needs urgent medical attention.
- Hemolysis (destruction of red blood cells) in people with G6PD deficiency, an inherited enzyme deficiency. The drug may also fail to work in these patients.
- Methemoglobinemia at high doses, as described above.
- Allergic reactions, which can be severe.
- Effects on pulse oximeters. The dye can make finger-clip oxygen readings temporarily inaccurate, which matters in hospital.
At high exposures, industrial and environmental sources report a range of harmful effects, which is another reason not to treat it casually.
Who should avoid methylene blue
Medical sources commonly advise that the following people should avoid methylene blue, or use it only under close supervision:
- People taking serotonergic medicines, including many antidepressants (SSRIs, SNRIs, MAO inhibitors, some tricyclics), and some other drugs that affect serotonin. Some clinicians also advise care with tyramine-rich foods, because methylene blue inhibits MAO.
- People with G6PD deficiency.
- People who are pregnant or breastfeeding. Methylene blue has been associated with harm to the fetus.
- Anyone who has had an allergic reaction to methylene blue or similar dyes.
- Children, unless it’s prescribed and supervised by a doctor.
People with kidney or liver problems, or other significant health conditions, should also seek medical advice first.
Lab stain, aquarium product or medicine?
Methylene blue comes in very different grades:
- Pharmaceutical grade is made and tested to strict standards for injection or oral use.
- Laboratory stain is made for colouring cells on slides. It may be a mixture, a different concentration, or a dye of variable purity. It isn’t intended for consumption.
- Aquarium treatments are intended for fish tanks and may contain other ingredients.
- Industrial dye can contain impurities, including metals left over from manufacturing.
A bottle of stain or aquarium product isn’t a medicine, even if it has the same name on the label.
If you use methylene blue as a microscope stain
Most readers of this site meet methylene blue as a stain for cheek cells, protozoa and bacteria. Used that way, in small amounts on slides, the main issues are practical:
- It stains skin, clothes and work surfaces, often permanently on fabric. Wear gloves and old clothes, and work on a tray or paper towel.
- Don’t swallow it, and keep it away from children. Label the bottle clearly and store it out of reach, away from food and drink.
- Avoid getting it in your eyes. If you do, rinse with plenty of water and follow the first-aid instructions on the product’s safety data sheet.
- If someone swallows stain, contact a poison control centre or a doctor for advice, especially if they take antidepressants or other regular medicines.
- Dispose of it as the supplier recommends. Small amounts of dilute stain are often rinsed away with lots of water, but local rules vary.
For the practical side of using it on slides, including dilution, cheek cell mounts and staining under the coverslip, see our methylene blue staining guide. It’s also one of the stains used to show nuclei in beginner histology.