Methylene Blue Dose Calculator
Methylene Blue Dose Calculator
Calculate a 1 or 2 mg/kg intravenous methylene blue dose for methaemoglobinaemia, with the volume of 1% solution, the repeat interval and the cumulative ceiling.
Methylene blue dose
Weight × mg/kg → mgA 70 kg adult given the usual initial dose of 1 mg/kg
Formula
Given intravenously over 5 minutes as a 1% solution — 0.1 mL/kg at 1 mg/kg, 0.2 mL/kg at 2 mg/kg
- 1 mg/kg
- the usual initial dose in acquired methaemoglobinaemia; 2 mg/kg is used in severe or refractory cases
- 1% solution
- 10 mg/mL, so the volume is 0.1–0.2 mL/kg; it is a vesicant and extravasation causes local necrosis
- repeat dose
- may be given after 30–60 minutes if methaemoglobin remains high, particularly with a long-acting trigger such as dapsone
- 7 mg/kg
- the approximate cumulative ceiling — beyond it methylene blue paradoxically causes methaemoglobinaemia and haemolysis
Worked example
A 70 kg adult given the usual initial dose of 1 mg/kg
70 × 1 = 70 mg of methylene blue
As a 1% solution (10 mg/mL) that is 7 mL, given intravenously over 5 minutes
At 2 mg/kg the same patient would receive 140 mg, or 14 mL of the 1% solution
Cumulative ceiling for this patient is roughly 7 mg/kg, or about 490 mg in total
Dose and volume of 1% solution by weight
| Weight | 1 mg/kg | Volume of 1% | 2 mg/kg | Volume of 1% |
|---|---|---|---|---|
| 50 kg | 50 mg | 5 mL | 100 mg | 10 mL |
| 60 kg | 60 mg | 6 mL | 120 mg | 12 mL |
| 70 kg | 70 mg | 7 mL | 140 mg | 14 mL |
| 80 kg | 80 mg | 8 mL | 160 mg | 16 mL |
| 100 kg | 100 mg | 10 mL | 200 mg | 20 mL |
Cautions before giving methylene blue
| Caution | Why it matters |
|---|---|
| G6PD deficiency | Ineffective, because the reducing pathway depends on NADPH, and it can precipitate haemolysis — use ascorbic acid or exchange transfusion |
| Cumulative dose above roughly 7 mg/kg | Paradoxically causes methaemoglobinaemia and haemolysis |
| Serotonergic drugs | Methylene blue is a monoamine oxidase inhibitor and can precipitate serotonin syndrome |
| Pregnancy | Crosses the placenta, with foetal harm in animal and case data — avoided unless the methaemoglobinaemia is itself life-threatening and no alternative is available |
| Interpreting the response | Blue discolouration interferes with co-oximetry and pulse oximetry for a period after the dose |
Dose, volume and the ceiling that catches people out
Methylene blue is the antidote for significant acquired methaemoglobinaemia. The usual initial dose is 1 mg/kg intravenously over 5 minutes, with 2 mg/kg reserved for severe or refractory cases. It is given as a 1% solution, which is 10 mg/mL, so the volume works out at 0.1 mL/kg at 1 mg/kg and 0.2 mL/kg at 2 mg/kg — 7 mL for a 70 kg adult at the usual dose. It is a vesicant, so the line should be checked before and during administration.
A response is normally seen within 30 minutes, and a repeat dose may be given after 30 to 60 minutes if methaemoglobin remains high. Repeat dosing is expected with a long-acting trigger such as dapsone, which outlasts a single dose comfortably. What has to be tracked is the cumulative total rather than the individual dose: above roughly 7 mg/kg methylene blue paradoxically causes methaemoglobinaemia itself and precipitates haemolysis, so the antidote starts to reproduce the problem it was given to treat.
Failure to respond should prompt a rethink rather than more drug. G6PD deficiency is the important one: methylene blue depends on NADPH generated by the pentose phosphate pathway, so in deficiency it is both ineffective and capable of precipitating haemolysis, and ascorbic acid or exchange transfusion is used instead. Continuing absorption of the trigger and sulfhaemoglobinaemia, which does not respond at all, are the other explanations to consider.
Two further cautions shape the decision. Methylene blue is a potent monoamine oxidase inhibitor, so giving it to a patient on serotonergic drugs — selective serotonin reuptake inhibitors, serotonin and noradrenaline reuptake inhibitors, tricyclics or triptans — carries a real risk of serotonin syndrome. In pregnancy it crosses the placenta and animal and case data show foetal harm, so it is avoided unless the methaemoglobinaemia is itself life-threatening and no alternative is available. Neither consideration necessarily withholds treatment from a critically poisoned patient, but both change the plan. Poisoning management is time-critical and directed by a poisons centre or clinical toxicology service, and this calculator supports that advice rather than replacing it.
Frequently asked questions
What is the dose of methylene blue for methaemoglobinaemia?
1 mg/kg intravenously over 5 minutes as a 1% solution is the usual initial dose, with 2 mg/kg in severe or refractory cases. A 1% solution is 10 mg/mL, so the volume is 0.1 to 0.2 mL/kg.
Can the dose be repeated?
Yes, after 30 to 60 minutes if methaemoglobin remains high, and repeat dosing is expected with long-acting triggers such as dapsone. Track the cumulative total, because above roughly 7 mg/kg methylene blue causes methaemoglobinaemia and haemolysis itself.
Why is methylene blue avoided in G6PD deficiency?
It relies on NADPH from the pentose phosphate pathway, so in deficiency it is ineffective, and it can precipitate haemolysis. Ascorbic acid or exchange transfusion is used instead.
Does methylene blue interact with antidepressants?
Yes. It is a monoamine oxidase inhibitor, so combining it with serotonergic drugs such as SSRIs, SNRIs, tricyclics or triptans risks serotonin syndrome. In pregnancy it is avoided unless the methaemoglobinaemia is itself life-threatening and no alternative is available, because it crosses the placenta and has caused foetal harm.
Who should the decision be made with?
A poisons centre or clinical toxicology service. Poisoning management is time-critical, and the cautions around G6PD deficiency, serotonergic drugs, pregnancy and cumulative dose all change the plan. This calculator supports that discussion and never replaces it.
Related calculators
References
- Ludlow JT, Wilkerson RG, Nappe TM. Methemoglobinemia. In: StatPearls. Treasure Island (FL): StatPearls Publishing.
- Provayblue (methylene blue) injection, for intravenous use — prescribing information. Provepharm Inc.
- Clifton J, Leikin JB. Methylene blue. Am J Ther. 2003;10(4):289–291.