METHYLENE BLUE (C16H18ClN3S)

Mechanism of Action
Methylene blue (methylthioninium chloride) acts as a redox agent and as an electron acceptor/donor at the cellular level. In the treatment of methemoglobinemia, methylene blue is reduced by the enzyme NADPH–methemoglobin reductase to leucomethylene blue, which donates electrons to convert methemoglobin (Fe³⁺) back into functional hemoglobin (Fe²⁺) capable of carrying oxygen. This redox cycle is sustained by regeneration of NADPH through the pentose phosphate pathway.
In addition, methylene blue can modulate mitochondrial respiration: it functions as an alternative electron carrier in the respiratory chain, accepting accumulated electrons (for example, when certain enzymes are dysfunctional) and facilitating ATP production. At higher doses, it also inhibits the nitric oxide (NO) pathway by binding to and inhibiting soluble guanylate cyclase in smooth muscle, preventing accumulation of cyclic GMP; this effect counteracts excessive NO-induced vasodilation. Methylene blue also reversibly inhibits monoamine oxidase A (MAO-A), which can increase neurotransmitter levels and carries a risk of serotonin syndrome when combined with antidepressants. In the nervous system, it has additionally been reported to inhibit glutamate production via nitric oxide synthase, reduce inflammasome activation (NLRP3/NLRC4), and act as an antioxidant with neuroprotective properties.
 
Clinical and Therapeutic Uses
Antidote for methemoglobinemia:
This is its most established clinical use. A 1% injectable solution (10 mg/mL) administered at 1–2 mg/kg IV rapidly reverses methemoglobinemia by fully restoring heme iron to its reduced state, thereby recovering hemoglobin’s oxygen-carrying capacity. Oral administration may be used in some cases. This treatment is life-saving in acquired methemoglobinemia due to toxins (e.g., nitrites, dapsone) or in symptomatic congenital forms.
 
Refractory vasoplegic shock (e.g., septic or post–cardiopulmonary bypass):
As adjunctive therapy, methylene blue helps reverse severe hypotension resistant to vasopressors. Its mechanism involves inhibition of guanylate cyclase and reduction of excessive NO signaling, increasing vascular resistance and blood pressure. Studies and clinical case reports describe rapid hemodynamic improvement with IV boluses of 1–2 mg/kg in vasodilatory shock of septic origin or following cardiac surgery.
 
Ifosfamide-induced neurotoxicity:
Methylene blue is used off-label for both prophylaxis and treatment of encephalopathy induced by ifosfamide (a chemotherapeutic agent). It prevents accumulation of the toxic metabolite chloroacetaldehyde by acting as an alternative electron acceptor, restoring mitochondrial function and blocking formation of neurotoxic metabolites. This has been shown to alleviate neurological symptoms and prevent recurrence in patients receiving ifosfamide.
 
Cyanide poisoning (adjunctive use):
Although standard cyanide protocols involve nitrites and thiosulfate, 1% methylene blue has been used as an additional measure in some cases. It may help by reversing therapeutically induced methemoglobinemia and counteracting NO-mediated vasodilation associated with the toxin. Administration must be immediate and under intensive monitoring.
 
Urinary tract infections and antisepsis:
Methylene blue has mild antiseptic properties in the urinary tract; historically it was included in preparations for urinary infections due to its bacteriostatic action and because it colors urine, facilitating monitoring. Its current use for UTIs is limited. Topically, in some countries methylene blue solutions are used to disinfect skin wounds and promote healing (astringent effect), though this is more traditional than evidence-based.
 
Diagnostic use (surgical dye):
A 1% methylene blue solution is used to mark tissues or anatomical pathways during procedures. For example, in oncologic surgery it is used to map the sentinel lymph node in breast cancer, helping identify the first draining lymph node. It is also used to delineate parathyroid glands in thyroid surgery, detect fistulas (e.g., via intravesical administration to visualize leaks), and test gastrointestinal patency. In laboratories, it is used as a histological stain and in microbiological assays (sample staining, Wright’s methylene blue, etc.).
 
Malaria (historical use):
Methylene blue was the first synthetic antimalarial successfully used in the late 19th century. Although it fell out of favor due to side effects (e.g., blue discoloration of urine and sclera), recent research has renewed interest in combining it with modern therapies, showing efficacy against parasite stages and reduced transmissibility. Its role in malaria still requires further investigation, but modern evidence suggests it may potentiate standard treatments.
 
Neurodegenerative disorders (proposed use):
Derivatives of methylene blue (such as LMTM or “Rember”) have been investigated as therapies for Alzheimer’s disease due to their ability to inhibit tau protein aggregation and improve neuronal mitochondrial function. However, clinical trials in patients with mild to moderate Alzheimer’s disease did not demonstrate significant cognitive benefit versus placebo. Its use has also been proposed in Parkinson’s disease and other dementias because of neuroprotective effects, but to date there is no conclusive evidence in humans.
 
Side Effects and Risks
When used at low therapeutic doses (1–2 mg/kg), methylene blue is generally well tolerated, but adverse effects may occur in a dose-dependent manner. Common side effects include headache, dizziness, nausea, sweating, and restlessness. Because of its intense dye properties, it may cause reversible cyanosis (bluish discoloration of skin, lips, or nails) and discolor urine to a blue-green or dark color for several hours after administration. A falsely low oxygen saturation reading on pulse oximetry is common immediately after injection, as the dye interferes with spectrophotometric measurements.
Serious risks include acute hemolytic anemia in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency. In such individuals, methylene blue cannot be effectively reduced to its leuco form, leading to accumulation and oxidative stress in red blood cells. For this reason, it is contraindicated in G6PD deficiency due to the risk of massive hemolysis and severe anemia. Excessive doses (>7 mg/kg cumulative) should also be avoided, as they may paradoxically induce rebound methemoglobinemia and hemolysis even in patients with normal G6PD activity.
Another major risk is serotonin syndrome, because methylene blue acts as a reversible MAO-A inhibitor. When administered together with SSRIs, SNRIs, or other serotonergic drugs, it can precipitate toxic serotonin levels. Cases of confusion, hyperthermia, hypertension, muscle rigidity, and even coma have been reported when methylene blue was inadvertently combined with these medications. Therefore, it is recommended to discontinue antidepressants at least two weeks before elective use of methylene blue and to avoid coadministration unless in emergency antidotal situations.