Methylene blue: dye, meaning and history

Methylene blue is a synthetic salt that appears as a dark green powder but dissolves to give a blue solution. First prepared in 1876, it is used as a dye in science and medicine, and as a treatment for methemoglobinemia and other conditions. Its colour changes depending on its chemical state, turning colourless when reduced.

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First made in 1876 by Heinrich Caro
Blue in solution, dark green as a powder
Used to treat methemoglobinemia
Acts as a redox indicator in chemistry
Turns colourless when reduced

What colour is Methylene blue?

Methylene blue is a laboratory-made dye that forms a blue solution in water, though its pure powder is dark green. Its colour is strong and clear, and it can change to colourless under reducing conditions, making it useful for chemical demonstrations. This dye is distinct from natural blues because it is fully synthetic and has unique redox properties.

Where does Methylene blue come from?

Heinrich Caro first prepared methylene blue in 1876. Its name comes from its blue colour in solution and its chemical structure, which is based on the phenothiazine group. The dye was quickly adopted in science and medicine for its vivid colour and ability to stain tissues and cells.

How is Methylene blue made?

Methylene blue is made by oxidising 4-aminodimethylaniline with sodium thiosulfate, followed by reaction with dimethylaniline and further oxidation and cyclization. The result is a salt that contains nitrogen, sulfur, and carbon. It is fully synthetic and not found in nature.

Human cheek cells stained with methylene blue. Photo: Joseph Elsbernd, CC BY-SA 2.0, via Wikimedia Commons

Where Methylene blue is used

This dye is widely used in medicine to treat methemoglobinemia and as a stain in surgery and laboratory tests. It colours tissues and fluids blue, helping doctors and scientists see structures more clearly. In chemistry, it acts as a redox indicator and a photosensitizer. It is also used in some textile and print applications.

Blue around the world

What the blue family means from country to country.

🇦🇫 Afghanistan

Lapis lazuli has been mined in the mountains of Badakhshan for more than six thousand years. Ground to powder it gave ultramarine, the costliest blue in European painting.

Royalty and power Sacred and holy Awe Source: Lapis lazuli

🇦🇫 Afghanistan

The chadari is a full-length covering for women with a mesh panel over the eyes. In Kabul and the north it is most often a light blue.

Protection and healing Source: Burqa

🇦🇲 Armenia

The blue band of the flag stands for the wish of the Armenian people to live under peaceful skies. The red above it is for the highlands and the struggle to survive.

Politics and protest Hope Source: Flag of Armenia

🇦🇷 Argentina

The flag is sky blue, white and sky blue, created by Manuel Belgrano in 1812. National teams are called the Albiceleste, the white and sky blue.

Politics and protest Pride Source: Flag of Argentina

🇦🇷 Argentina

Boca Juniors play in blue and gold. The club's story is that in 1907 it took the colours of the flag of the next ship into port, which was Swedish.

Festival and celebration Pride Source: Boca Juniors

🇦🇺 Australia

To be true blue is to be honestly and loyally Australian. The phrase is older than the country, but Australians have made it their own.

Politics and protest Trust Source: True Blue (John Williamson song)

Shades and relatives of Methylene blue

When methylene blue is oxidized or reduced, its colour changes: it is blue when oxidized and colourless when reduced to leucomethylene blue. Related dyes include Azure A, Azure B, Azure C, and thionine, which are formed by demethylation of methylene blue. These are used in various staining techniques.

Questions people ask about Methylene blue

What colour is methylene blue?

Methylene blue is a dark green powder that gives a blue solution in water. Its blue colour is strong and clear, but it can turn colourless when chemically reduced.

Where does the name methylene blue come from?

The name refers to its blue colour in solution and its chemical structure. It is also known as methylthioninium chloride.

How is methylene blue made?

It is made by oxidising 4-aminodimethylaniline, reacting it with dimethylaniline, and then further oxidising and cyclizing the product. The process uses nitrogen, sulfur, and carbon.

What is methylene blue used for?

It is used as a dye in medicine and science, to treat methemoglobinemia, as a stain in surgery and lab tests, and as a redox indicator and photosensitizer in chemistry.

Is methylene blue natural or synthetic?

Methylene blue is a fully synthetic dye. It does not occur in nature and is made through a chemical process.

Good reading and sources

Where this page comes from

The facts on this page come from the Wikipedia article "Methylene blue", written by its contributors and used under the CC BY-SA 4.0 licence, and from its Wikidata record (CC0). Rang rewrote the text in its own words, with AI assistance, and compares every page with the article to make sure no passage is copied. It may be reused under the same CC BY-SA 4.0 licence with a link back.

It may contain mistakes. The swatch is an approximation: named colours are not fixed standards, and every screen shows them a little differently. If something is wrong, tell us. Last refreshed 2026-10-03.

Methylene blue