Methylene Blue

How it works

It acts as an electron-transfer cofactor for NADPH-methaemoglobin reductase, reducing the ferric iron of methaemoglobin back to the functional ferrous form so haemoglobin can again carry oxygen.

Prescribing in practice

• Avoid in patients taking serotonergic drugs where possible, as methylene blue has potent monoamine oxidase inhibitory activity and can precipitate serotonin syndrome.
• It is contraindicated or used with extreme caution in G6PD deficiency, where it is ineffective and may itself cause haemolysis.
• Give by slow intravenous injection; rapid or excessive administration can paradoxically worsen methaemoglobinaemia.

Monitoring

Monitor methaemoglobin levels, oxygen saturation (noting pulse oximetry is unreliable in methaemoglobinaemia, so co-oximetry is preferred) and clinical response after administration.

Counselling the patient

• Warn the patient that urine, saliva and the skin may turn blue-green for a day or two.
• Advise the patient to report new agitation, fever or muscle twitching, which could indicate serotonin syndrome.
• Explain the treatment reverses the abnormal blood pigment causing breathlessness or blue discolouration.

Evidence & guidelines

Methylene blue is established as the standard antidote for symptomatic acquired methaemoglobinaemia, with the MHRA having highlighted the serotonin syndrome interaction risk.