October 2025

Methylene Blue: From Antimicrobial Effects to Cancer Treatment

The antimicrobial and anti-psychotic activities of methylene blue (MB) have been known since the late 1800’s (reviewed in Kwok et al 2006 and Yang et al 2017), and since then much has been learned about the activities of methylene blue in cells. This work has revealed that MB is a powerful inhibitor of the mitochondrial enzyme monoamine oxidase. Monoamine oxidases inactivate neurotransmitters such as norepinephrine, dopamine, and serotonin as part of normal turnover activities in neurons, and interactions with those pathways presumably explain the neurotropic effects of the molecule.  MB also depresses synthesis of nitric oxide, an important vasodilator (Mayer 1993; reviewed in Saha and Burns 2020). It is also known that MB modulates the electron transport chain in the mitochondria, transferring electrons to cytochrome c which then hands them off directly to the cytochrome c oxidase (outlined in Tucker et al 2018). This short circuiting of the electron transport chain is thought to facilitate electron transport in situations where the upstream carriers are compromised (as occurs in damaged or cyanide poisoned mitochondria), but probably slows down ATP synthesis in healthy mitochondria. MB is also used extensively in surgery to delineate tissues, especially in delimiting cancerous from normal tissue at the borders of tumors, and has also been explored as an antibacterial agent for potential use in treating clinical infections (Li et al 2016).

There has been growing interest in the possibility of utilizing methylene blue as an adjuvant (treatment that accompanies or follows behind standard chemotherapy). Yang and colleagues (2017) review a number of studies into the anti-cancer activities of methylene blue and point out that results are preliminary but promising. Efforts do not yet appear to have progressed to clinical trial stage, but many interesting preclinical studies have been conducted.  A 2006 study by Wondrak reported apoptosis (cell suicide) of metastatic melanoma lines LOX and G361 when treated with micromolar concentrations of methylene blue or toluidine blue. Identical conditions eliminated pancreatic carcinoma cells but had no effect on untransformed human skin cells and normal CF3 fibroblasts. The author also observed that a human breast carcinoma line MDA-MB23 and A375 melanoma were resistant to the same treatments, and identified a cellular inhibitor expressed in many tumors, NQO1, that negates methylene blue’s antitumor activity in vitro. This indicates that any anti-tumor activities of methylene blue are going to be dependent on the mutation background of any given tumor in any given patient. A more recent study that examined light activation of methylene blue (referred to as photodynamic treatment) with a carcinosarcoma (rare cancer with cells from both epithelial and connective tissues) model in rats suggested that 0.1% methylene blue plus 148J/cm light exposure has significant anti-tumor activity and led to enhanced survival (Petrellis 2019). Photodynamic activation of MB has been studied in human breast cancer cell lines, revealing that MB has in combination with light has substantial suppressive activity on cancer cells in vitro and only a modest effect on normal cells in culture (Santos 2017).  Photodynamic methylene blue combination therapy has also been used in clinical settings (Tardivo et al 2005). With interest in using MB systemically to treat cancer, the question of whether MB can cause instead of cure cancer has been raised, so it is worth noting that although MB exhibits mutagenic potential (genotoxicity) in Ame’s assays, it appears NOT to be carcinogenic in mice and rats at therapeutic doses (Auerbach 2010).

While widespread systemic (body-wide) effects of MB on the human body are largely unknown, in general MB appears to be well tolerated in humans at therapeutic doses in clinical settings (Buzga et al 2022). It should be noted that adverse reactions have been reported. For example, there are well-known serious adverse interactions between methylene blue and the serotonin reuptake inhibitors (SSRI) used to treat depression and bi-polar disorders - this occurs at MB doses as low as 1mg/kg. The toxic effects of the interactions occur in the central nervous system (Gillman 2011). Methylene Blue is also known to exhibit toxic effects at therapeutic doses in individuals with glucose-6-phosphate dehydrogenase deficiencies and kidney disease (Buzga et al 2022). Evora et al (2015), who describe long experience using MB, point out that it decreases arterial oxygenation, likely through inhibition of nitric oxide synthesis which, as described above, prevents normal vasodilation (blood vessel expansion); this effect counter-indicates MB for patients in respiratory distress. MB has also been shown to induce anemia and other metabolic disturbances to the fetus in utero and in neonates (Crooks 1982, Porat et al 1996). Finally, its antimicrobial activities have raised the possibility that gut dysbiosis (disruption of the microbiome) might result from MB use. To address this possibility, Gureev et al (2020) used a mouse model to show that doses up to 40mg/kg had no effect on the microbial assemblage of the laboratory mice. Others have shown that MB can prevent dysbiosis in mice treated with the anti-tumor agent cisplatin (Krutskikh et al 2022), but at this juncture the potential of methylene blue to cause gut dysbiosis in humans has not been studied.

Overall, with the exception of the few cautionary situations described above, MB appears to be safe and possibly useful in treating some cancers. We look forward to future experimental work to provide a better understanding of mechanisms, which will help understand the types of tumors that respond to MB treatment.

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Bužga, M., Machytka, E., Dvořáčková, E., Švagera, Z., Stejskal, D., Máca, J., & Král, J. (2022). Methylene blue: a controversial diagnostic acid and medication?. Toxicology Research, 11(5), 711-717.

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