Methylene Blue

How it works

It inhibits guanylate cyclase and nitric oxide synthase, reducing cyclic GMP-mediated vasodilatation to restore vascular tone; in methaemoglobinaemia it acts via NADPH-dependent reduction to convert methaemoglobin back to functional haemoglobin.

Prescribing in practice

• It is a potent serotonergic agent and can precipitate serotonin syndrome when combined with serotonergic drugs such as SSRIs, SNRIs and MAOIs, which should be reviewed before administration.
• It is contraindicated or hazardous in glucose-6-phosphate dehydrogenase deficiency, where it is ineffective for methaemoglobinaemia and may cause haemolysis.
• It transiently interferes with pulse oximetry readings and discolours urine blue-green; give slowly intravenously as it can itself cause methaemoglobinaemia at high cumulative doses.

Monitoring

Monitor blood pressure, oxygenation (recognising pulse oximetry interference) and, where relevant, methaemoglobin levels and for signs of serotonin toxicity.

Counselling the patient

• This medicine may turn the urine a blue-green colour, which is harmless and expected.
• The team will review all other medicines first because of an interaction risk with certain antidepressants.
• Oxygen saturation readings may read inaccurately for a short time after it is given.

Evidence & guidelines

Methylene blue is the established first-line treatment for acquired methaemoglobinaemia and is used as adjunctive rescue therapy in refractory vasoplegia; MHRA advises caution because of its serotonergic interaction potential.