Key takeaways
- No human study has measured what methylene blue does to the gut microbiome. The evidence is two mouse studies from one research group.
- In young mice, a lower dose (15 mg/kg a day for four weeks) caused no significant shift toward an unhealthy microbiome, though a few species fell, including two Lactobacillus species. A higher dose (50 mg/kg) raised Proteobacteria to 7.49% against 1.61% in controls.
- Even the lower mouse dose works out to roughly 85 mg a day for a 70 kg adult using the standard dose conversion, many times a 5 mg capsule.
- What people notice in practice: urine and stool can turn green or blue, which is expected. Take NooBlue capsules with food, as the label says.
- Not with SSRIs, SNRIs, MAOIs or other serotonergic medicines, not with G6PD deficiency, not in pregnancy or breastfeeding, and not under 18.
Questions about methylene blue and gut health usually start from a fair worry. Methylene blue kills microbes in lab experiments, so what does a daily dose do to the bacteria in your gut? The short answer is that nobody has studied it in people. What exists are two mouse studies from the same university group, a handful of known side effects from medical use, and a lot of confident advice online that goes well past the data.
This guide sets out what those mouse studies found, what their doses mean next to a supplement serving, the gut effects you might actually notice, and which popular claims (killing gut parasites, probiotic timing rules, “die-off”) have no evidence behind them. NooBlue sells methylene blue, so we have an interest here. Every study is linked so you can check our reading.
Why methylene blue raises gut questions
Methylene blue is active against microorganisms from many groups, and its antibacterial effect is best studied in photodynamic therapy, where light activates the dye to produce reactive oxygen species that damage target cells. The authors of the main mouse study below set this out in their introduction, and noted that before their work nobody had tested what methylene blue does to the intestinal microbiome of lab animals.
Two things follow. The germ-killing results people quote mostly come from light-activated or topical use, which is not what happens inside a dark gut. And because oral methylene blue is well absorbed in the upper gut (Walter-Sack and colleagues measured 72.3% absolute bioavailability in 16 healthy adults), it is not obvious how much reaches the colon at supplement doses. Nobody has measured that either.
What the mouse microbiome studies found
Gureev and colleagues, PLoS One, 2020
Researchers at Voronezh State University gave three-month-old C57Bl/6 mice methylene blue for four weeks at 15 mg/kg a day (9 mice) or 50 mg/kg a day (8 mice), with 13 mice on plain water as controls. They sequenced the gut bacteria in droppings each week and tested the mice in a T-maze at the end.
- Lower dose: no significant changes that pointed to dysbiosis. There were smaller shifts: the Bacteroidetes to Firmicutes ratio fell in weeks two and three, Actinobacteria dipped in week three, and a few species dropped, including Lactobacillus murinus and Lactobacillus coleohominis.
- Higher dose: clearer changes, led by a rise in Proteobacteria. By week four the median Proteobacteria share was 7.49%, against 1.61% in the control group.
- Behavior: the lower dose improved scores on the T-maze alternation test. The higher dose did not significantly improve them, and the authors suggested the microbiome shift might be one reason.
The authors were careful about what this shows. They noted that the correlations between bacteria and behavior don’t prove a direct link, that the T-maze alone can’t fully assess cognition, and that mouse findings can’t be carried straight over to people.
Sadovnikova and colleagues, Pharmaceuticals, 2021
The same group tested methylene blue, dimethyl fumarate and resveratrol in 15-month-old mice. All three improved short- and long-term memory, which the authors linked to mitochondrial quality control in the brain. Long-term methylene blue lowered gut microbiome diversity, but the other markers of dysbiosis they looked for did not appear.
That is the whole animal evidence base on methylene blue and the microbiome that we could find in a PubMed search in September 2026. Two studies, one lab, mice only.
What those doses mean for a person
Mouse doses can’t be compared with human doses by weight alone, because small animals clear drugs faster. The usual shortcut is the body-surface-area conversion described by Nair and Jacob (Journal of Basic and Clinical Pharmacy, 2016): multiply a mouse dose in mg/kg by 0.081 to get a rough human-equivalent dose.
| Mouse dose | Rough human equivalent | For a 70 kg adult | What the mice showed |
|---|---|---|---|
| 15 mg/kg a day | About 1.2 mg/kg | About 85 mg a day | No significant dysbiosis, some species shifts |
| 50 mg/kg a day | About 4 mg/kg | About 280 mg a day | Proteobacteria up, no memory gain |
A NooBlue capsule holds 5 mg and a gummy 10 mg, taken once a day. Both sit well below even the lower mouse dose. That is reassuring, but it is not proof of safety for your gut. The conversion is rough, mouse and human guts differ, and a lower dose simply hasn’t been tested. We’d put it this way: the one controlled look at the microbiome found the bigger changes at a dose far above supplement servings, and nobody has looked at supplement servings at all.
For the dose side of methylene blue more generally, including the safety thresholds in medical use, see our guide to methylene blue benefits and side effects.
Gut effects you might notice
- Green or blue urine and stool. MD Anderson’s Dr. Carlos Roldan notes that swallowed methylene blue can turn urine and stool green until it is out of your system, and that this is normal. Our guide to methylene blue urine color explains why.
- Nausea and diarrhea. The StatPearls chapter on methylene blue lists both among the most commonly reported reactions in medical use, where doses are much higher than supplement servings. NooBlue capsules say to take one daily with food. The best time to take methylene blue covers timing and meals.
- A blue mouth or tongue. This comes from drops touching your mouth, not from anything in your gut. Capsules are swallowed and the gummies don’t leave a blue mouth.
Severe stomach pain, vomiting that won’t stop, blood in your stool, or yellowing of your skin or eyes are not expected effects. Stop taking it and get medical help.
Probiotics, fermented foods and diet
Online guides often tell you to take probiotics four hours away from methylene blue, keep vitamin C two hours apart, or stop a week before a stool test. We could not find a study behind any of those rules. If spacing a probiotic from your methylene blue gives you peace of mind, it costs nothing, but nobody has shown it matters.
Vitamin C is a special case because it can shift methylene blue toward its colorless reduced form. NooBlue gummies contain 25 mg of vitamin C and the capsules 10 mg, by design, so a normal multivitamin needs no separate slot.
Food is a more reasonable question. Ramsay, Dunford and Gillman (British Journal of Pharmacology, 2007) showed methylene blue is a potent reversible inhibitor of MAO-A and noted this has implications for how the gut handles dietary amines when it is taken by mouth. We found no reports of food reactions at supplement doses. If a doctor has ever told you to limit tyramine-rich foods because of a medicine you take, ask them before adding methylene blue, and read our list of what not to take with methylene blue.
Does methylene blue kill parasites in the gut?
There is no evidence that a supplement dose does. The parasite claims usually borrow from two other uses. Aquarium methylene blue is used against fungus and external protozoa on fish; Kordon’s product, for example, is a 2.303% solution and its maker states it is not intended for human consumption. And methylene blue has a long record as an antimicrobial dye in lab and light-activated use. Neither tells you anything about a 5 mg capsule in the human intestine. If you think you have a gut parasite, see a doctor for testing and a targeted medicine.
Choosing a format if your stomach is sensitive
| Format | Serving | Gut-related notes | Price (as of Sep 2026) |
|---|---|---|---|
| NooBlue Capsules | One 5 mg capsule a day | Label says take with food. The smallest fixed dose we sell. | $37.99 for 60 ($0.63 a day) |
| NooBlue Gummies | One 10 mg gummy a day | Chewed, with 25 mg vitamin C. No blue mouth. | $49.99 for 60 ($0.83 a day) |
| NooBlue Drops 1% | About 0.5 mg per drop, 10 drops = 5 mg | Can be mixed into water or juice. Can stain mouth and tongue. | $29.99 for 50 mL, about 100 servings ($0.30 each) |
If your stomach is the concern, we’d start with the capsules taken with a meal. They give a fixed 5 mg and follow the label’s with-food instruction. Drops let you go lower than 5 mg, but counting drops takes care, and you should never exceed the suggested serving. For how absorption differs between forms, see methylene blue bioavailability in liquid vs capsules, and for other capsule brands, our methylene blue capsule comparison. Free worldwide shipping starts at $100 on the NooBlue shop page.
Who should not take methylene blue
Safety first
- Do not take methylene blue with SSRIs, SNRIs, MAOIs or other serotonergic medicines. The combination carries a risk of serotonin syndrome.
- Do not take it if you have G6PD deficiency.
- Do not take it if you are pregnant or breastfeeding.
- Not for anyone under 18.
- It can turn urine blue or green. That is expected.
- Talk to your doctor before use if you take any prescription medicine or have a kidney or liver condition.
If you live with an ongoing gut condition or take medicine for one, talk to your doctor before trying methylene blue. Our guide to whether methylene blue is safe to take daily covers long-term use, and is methylene blue good for you? weighs the evidence overall. For the full picture of what the research supports, see our methylene blue benefits guide.
Frequently asked questions
Does methylene blue kill good gut bacteria?
Nobody has tested this in people. In mice, 15 mg/kg a day for four weeks caused no significant shift toward dysbiosis, though a few species fell, including two Lactobacillus species. At 50 mg/kg, Proteobacteria rose clearly. Both doses are far above a supplement serving after conversion.
Why is my stool green after taking methylene blue?
Swallowed methylene blue can turn stool and urine green or blue until it clears your system. That is expected. Black or bloody stool is different and needs a doctor.
Is methylene blue killing parasites?
There’s no evidence that supplement doses kill gut parasites in people. Aquarium methylene blue is used on fish and is labeled not for human consumption. If you suspect a parasite, get tested by a doctor.
What foods should you avoid when taking methylene blue?
No food list has been set for supplement doses, and we found no reports of food reactions. Methylene blue inhibits MAO-A, which researchers say could affect how the gut handles dietary amines. If you’ve been told to limit tyramine-rich foods because of a medicine, ask your doctor first.
Does methylene blue flush toxins out of your body?
No. Blue or green urine is the dye itself leaving your body, not toxins. No study shows methylene blue clears toxins at supplement doses.
Why do I feel so good on methylene blue?
We can’t say for certain, and no study has looked at mood in healthy people at supplement doses. It inhibits MAO-A, the enzyme that breaks down serotonin, which is also why it is dangerous with antidepressants. Expectation plays a part with any new supplement.
Can I take probiotics with methylene blue?
We found no study on the combination and no evidence for the common “four hours apart” rule. If you want to space them, it does no harm.
Are methylene blue “die-off” symptoms real?
We found no study showing die-off reactions from methylene blue at supplement doses. Headache, nausea or feeling unwell after a dose is a reason to stop and check with your doctor, not to push through.
Is methylene blue hard on the liver?
We found no established pattern of liver problems at supplement doses, but it hasn’t been well studied. The label says to talk to your doctor before use if you have a kidney or liver condition.
Sources
- Gureev AP, et al. Effect of long-term methylene blue treatment on the composition of mouse gut microbiome and its relationship with the cognitive abilities of mice. PLoS One. 2020;15(11):e0241784. PubMed 33206681 (full text)
- Sadovnikova IS, et al. Nrf2/ARE activators improve memory in aged mice via maintaining of mitochondrial quality control of brain and the modulation of gut microbiome. Pharmaceuticals. 2021;14(7):607. PubMed 34201885
- Nair AB, Jacob S. A simple practice guide for dose conversion between animals and human. J Basic Clin Pharm. 2016;7(2):27-31. PubMed 27057123
- Walter-Sack I, et al. High absolute bioavailability of methylene blue given as an aqueous oral formulation. Eur J Clin Pharmacol. 2009;65(2):179-189. PubMed 18810398
- Ramsay RR, Dunford C, Gillman PK. Methylene blue and serotonin toxicity: inhibition of monoamine oxidase A (MAO A) confirms a theoretical prediction. Br J Pharmacol. 2007;152(6):946-951. PubMed 17721552
- Ostrovsky A, Afzal M. Methylene Blue. StatPearls, updated January 2026. NCBI Bookshelf NBK557593
- MD Anderson Cancer Center. Methylene blue: Should you take it? March 2026. mdanderson.org
- Kordon. Methylene Blue (aquarium product page). kordon.com
