- Serotonin syndrome is the most serious documented risk, especially for anyone taking SSRIs, SNRIs, or MAOIs — this combination can be fatal[1]
- Methylene blue is contraindicated for people with G6PD deficiency, since it can trigger a hemolytic reaction[1]
- Green or blue-tinted urine and stool is the most common (harmless) complaint, alongside GI upset and bladder irritation[2]
- Most methylene blue sold as a “supplement” isn’t in the federal dietary-supplement label registry, a real regulatory gray area flagged by NPR[3]
Methylene blue has moved from a niche prescription dye and antidote into a wellness-supplement trend, and the negative-review picture reflects that mismatch: several of the most serious warnings come from its long medical history as a drug, not from casual supplement complaints.
The Most Serious Risk: Serotonin Syndrome
Across medical and drug-reference sources, the most significant concern is serotonin syndrome — a reaction that can cause elevated blood pressure, diarrhea, seizures, and death. People taking certain antidepressants, including Zoloft and Lexapro, are especially at risk[1]. Drug references carry an explicit warning that methylene blue may cause serious or fatal serotonergic syndrome when combined with serotonergic drugs, and concomitant use with SSRIs, SNRIs, and MAOIs should be avoided entirely[1].
G6PD Deficiency: A Contraindication, Not a Caution
Methylene blue is specifically contraindicated in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency, since it can trigger a hemolytic response — intravascular hemolysis that typically occurs two to five days after administration[1]. Anyone unaware of their G6PD status should treat this as a real reason to consult a doctor before use, not a boilerplate disclaimer.
Common, Milder Complaints
The most frequently reported effect, even at supplement-level doses, is discoloration: methylene blue can turn urine and stool green or blue until it clears the system, which is considered normal and harmless[2]. Clinical data also documents bladder irritation, urinary frequency, diarrhea or loose stool, and nausea or vomiting as common adverse events[2]. In oral rinse formulations, about 5% of first-time users reported a burning sensation that typically resolved by the second or third use[2].
Overdose Risk Is Real, Not Theoretical
At higher intravenous doses (7 mg/kg or more), documented effects include nausea, vomiting, chest pain, difficulty breathing, tachycardia, tremor, and confusion. Severe overdosage — a single dose of 20 mg/kg or more — has caused severe intravascular hemolysis, hyperbilirubinemia, and death[1]. While these figures come from clinical/IV contexts, they underscore that this is a compound with a real dose-toxicity curve, not a benign wellness additive.
The Regulatory Gray Area — A Complaint in Itself
NPR’s reporting highlighted a structural problem: most methylene blue products sold online are not in the federal registry of dietary supplement labels, and many aren’t explicitly identified as supplements at all. The FDA reportedly did not respond to questions about how retailers can sell it without a prescription[3]. Experts recommend anyone considering it talk to a clinician first to rule out contraindications like SSRI use or G6PD deficiency[3].
Drug Interaction Beyond Antidepressants
Because methylene blue is metabolized via cytochrome P450 pathways, it can interact with medications including alfentanil, cyclosporine, digoxin, fentanyl, phenytoin, sirolimus, tacrolimus, and warfarin[1].
FAQ
Can methylene blue be taken with antidepressants?
Combining it with SSRIs, SNRIs, or MAOIs carries a serious risk of serotonin syndrome, which can be fatal, and should be avoided[1].
Is methylene blue safe for everyone?
No — it’s specifically contraindicated for people with G6PD deficiency[1], and its supplement-market regulatory status remains murky[3].
Why does methylene blue turn urine green or blue?
This is a normal, harmless discoloration effect and the most commonly reported response, not a sign of harm[2].
References
- Drugs.com. Methylene Blue Side Effects: Common, Severe, Long Term. www.drugs.com/sfx/methylene-blue-side-effects…
- Mayo Clinic. Methylene blue (intravenous route) – Side effects & uses. www.mayoclinic.org/drugs-supplements/methylen…
- NPR. What’s behind the wellness claims for the synthetic dye methylene blue?. www.npr.org/2025/12/15/nx-s1-5634680/methylen…
These statements have not been evaluated by the Food and Drug Administration. This information is not intended to diagnose, treat, cure, or prevent any disease. Content is for informational purposes only and is not medical advice; consult a qualified healthcare provider before starting any supplement. As an Amazon Associate we earn from qualifying purchases.