Antidote / dye

Methylthioninium chloride (methylene blue)

Brand names: Provayblue

Adult dose

Dose: Methaemoglobinaemia: 1–2 mg/kg IV slow over 5 min; repeat after 30–60 min if needed (max 7 mg/kg). Vasoplegia (refractory): 1–2 mg/kg IV

Route: IV

Frequency: PRN

Clinical pearls

Drug-induced methaemoglobinaemia (e.g. dapsone, prilocaine, nitrites) when SpO2 reading misleadingly low
Counsel re harmless blue urine

Contraindications

G6PD deficiency (haemolysis)
Severe renal impairment
Hypersensitivity
Concurrent serotonergic drugs (serotonin syndrome — caution)

Side effects

Blue/green discolouration of urine and skin
Dyspnoea
Hypertension
Methaemoglobinaemia at high doses (paradoxical)
Hypersensitivity

Interactions

SSRIs/SNRIs/MAOIs/triptans (serotonin syndrome)

Monitoring

Methaemoglobin level
ABG / co-oximetry
BP

Reference: BNF; TOXBASE; NPIS; https://bnf.nice.org.uk/drugs/methylthioninium-chloride/. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Pathways