Methylthioninium chloride (methylene blue)

How it works

It serves as an electron carrier for NADPH-methaemoglobin reductase, accelerating the reduction of ferric methaemoglobin back to oxygen-carrying ferrous haemoglobin.

Prescribing in practice

• Because it is a potent monoamine oxidase inhibitor, avoid co-administration with serotonergic agents where possible owing to the risk of serotonin syndrome.
• It is ineffective and potentially haemolytic in G6PD deficiency and should be avoided in that setting.
• Administer by slow intravenous injection, as excessive doses can themselves induce methaemoglobinaemia.

Monitoring

Monitor methaemoglobin concentration by co-oximetry, oxygen saturation and clinical recovery following the dose.

Counselling the patient

• Forewarn the patient that urine and other secretions may appear blue or green temporarily.
• Ask the patient or team to watch for agitation, hyperthermia or tremor suggesting serotonin syndrome.
• Reassure that the agent treats the underlying abnormal blood pigment rather than just symptoms.

Evidence & guidelines

It is the recognised standard treatment for clinically significant acquired methaemoglobinaemia, with the SPC and MHRA advice noting the serotonergic interaction.