[12] Since its reduction potential is similar to that of oxygen and can be reduced by components of the electron transport chain, large doses of methylene blue are sometimes used as an antidote to potassium cyanide poisoning, a method first successfully tested in 1933 by Matilda Moldenhauer Brooks in San Francisco,[15] although first demonstrated by Bo Sahlin of Lund University, in 1926.

[15][16] Methylene blue is used in endoscopic polypectomy as an adjunct to saline or epinephrine, and is used for injection into the submucosa around the polyp to be removed.

Methylene blue is also used as a dye in chromoendoscopy, and is sprayed onto the mucosa of the gastrointestinal tract in order to identify dysplasia, or pre-cancerous lesions.

Intravenously injected methylene blue is readily released into the urine and thus can be used to test the urinary tract for leaks or fistulas.

[citation needed] In surgeries such as sentinel lymph node dissections, methylene blue can be used to visually trace the lymphatic drainage of tested tissues.

On the other hand, if methylene blue is used it might help to give the normal look of neutrophil granules and may also enhance the staining of nucleoli and polychromatophilic RBCs (reticulocytes).

[20] A traditional application of methylene blue is the intravital or supravital staining of nerve fibers, an effect first described by Paul Ehrlich in 1887.

The selective blue coloration develops with exposure to air (oxygen) and can be fixed by immersion of the stained specimen in an aqueous solution of ammonium molybdate.

[22][23][24] The mechanism of selective dye uptake is incompletely understood; vital staining of nerve fibers in skin is prevented by ouabain, a drug that inhibits the Na/K-ATPase of cell membranes.

[25] Methylene blue has been used as a placebo; physicians would tell their patients to expect their urine to change color and view this as a sign that their condition had improved.

[26] This same side effect makes methylene blue difficult to use in traditional placebo-controlled clinical studies, including those testing for its efficacy as a treatment.

A toxic metabolite of ifosfamide, chloroacetaldehyde (CAA), disrupts the mitochondrial respiratory chain, leading to an accumulation of nicotinamide adenine dinucleotide hydrogen (NADH).

[33] Methylene blue increases blood pressure in people with vasoplegic syndrome (redistributive shock), but does not improve delivery of oxygen to tissues or decrease mortality.

[34][35] Methylene blue has been used in calcium channel blocker toxicity as a possible rescue therapy for distributive shock unresponsive to first line agents.

Limited to case reports, a 2024 review found low-quality evidence that methylene blue may reduce short-term mortality, duration of the need for vasopressors, and length of hospital stay.

It is used in this regard to make organic peroxides by a Diels-Alder reaction which is spin forbidden with normal atmospheric triplet oxygen.

The methylene blue sulfide test is a convenient method often used in soil microbiology to quickly detect in water the metabolic activity of sulfate reducing bacteria (SRB).

[50] The addition of a strong reducing agent, such as ascorbic acid, to a sulfide-containing solution is sometimes used to prevent sulfide oxidation from atmospheric oxygen.

[46] Methylene blue is a dye behaving as a redox indicator that is commonly used in the food industry to test the freshness of milk and dairy products.

A color reaction in an acidified, aqueous methylene blue solution containing chloroform can detect anionic surfactants in a water sample.

[54] In materials science, methylene blue solution is successively added to fine aggregate which is being agitated in water.

During this period before the first World War, researchers like Ehrlich believed that drugs and dyes worked in the same way, by preferentially staining pathogens and possibly harming them.