Methylene Blue
Methylene blue can reduce methemoglobin to functional hemoglobin. At excessive exposure it instead contributes to oxidative red-cell injury.
Group
Methemoglobinemia antidote
Mechanism of Action
Reduced leucomethylene blue transfers reducing equivalents to methemoglobin through an NADPH-dependent pathway.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Methemoglobinemia | 1.5 mg/kg | IV | Once | Give slowly. Cats are especially susceptible to oxidative red-cell injury; acetylcysteine is preferred for acetaminophen poisoning. |
| Phenol-associated methemoglobinemia | 4 mg/kg | IV | Once | Slow infusion; oral ascorbic acid 20 mg/kg has been described as an adjunct. |
| Alternative severe methemoglobinemia rescue | 1 mg/kg | IV | Once | Give slowly over several minutes; reassess within 30 minutes and monitor hematocrit for 3 days before considering further exposure. |
| Intraoperative pancreatic islet tumor staining | Reference only | IV | 3 mg/kg in 250 mL sterile normal saline infused over 30–40 minutes intraoperatively. Staining may begin after about 20 minutes and peak at 25–35 minutes. The described dilution volume and timing require a surgical plan; do not infer a universal fluid volume from weight. |
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Methemoglobinemia | 1.5 mg/kg | IV | Once | Give slowly. Cats are especially susceptible to oxidative red-cell injury; acetylcysteine is preferred for acetaminophen poisoning. Routine feline use is discouraged; this is a selected rescue alternative after explicit assessment of oxidative hemolysis risk. |
| Alternative single-dose methemoglobinemia rescue | 1–1.5 mg/kg | IV | Once | One dose only. Routine feline use is discouraged because oxidative hemolysis can worsen; use only after explicit rescue assessment. |
Cattle
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Methemoglobinemia | 4–10 mg/kg | IV | As needed | Infuse over 15 minutes; subsequent lower doses are titrated to response. Reassess before repeating. |
| Severe methemoglobinemia | 15–20 mg/kg | IV | Every 6–8 h | Severe ruminant toxicity only; not the routine initial range. Administer slowly and reassess red-cell injury and methemoglobin before any repeat; higher dosing increases oxidative risk. |
| Alternative oxidant methemoglobinemia regimen | 4–15 mg/kg | IV | Every 6 h | Described with 1% medicinal solution for nitrate/nitrite/chlorate exposure. Reassess hemolysis and response before repeating; alternative to other listed ranges. |
| Historical cyanide antidote adjunct | Reference only | IV | An older adjunctive amount is 4–6 g per 454 kg body weight IV. Use only within a complete selected cyanide emergency protocol; methylene blue is not a substitute for oxygenation and an appropriate cyanide antidote. | |
| Nitrate-associated methemoglobinemia | 5–15 mg/kg | IV | Once | 1% medicinal solution in physiologic saline; severe cases may need a lower repeat amount after reassessment. |
Goats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Methemoglobinemia | 4–10 mg/kg | IV | As needed | Infuse over 15 minutes; subsequent lower doses are titrated to response. Reassess before repeating. |
| Severe methemoglobinemia | 15–20 mg/kg | IV | Every 6–8 h | Severe ruminant toxicity only; not the routine initial range. Administer slowly and reassess red-cell injury and methemoglobin before any repeat; higher dosing increases oxidative risk. |
| Alternative oxidant methemoglobinemia regimen | 4–15 mg/kg | IV | Every 6 h | Described with 1% medicinal solution for nitrate/nitrite/chlorate exposure. Reassess hemolysis and response before repeating; alternative to other listed ranges. |
| Historical cyanide antidote adjunct | Reference only | IV | An older adjunctive amount is 4–6 g per 454 kg body weight IV. Use only within a complete selected cyanide emergency protocol; methylene blue is not a substitute for oxygenation and an appropriate cyanide antidote. |
Sheep
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Methemoglobinemia | 4–10 mg/kg | IV | As needed | Infuse over 15 minutes; subsequent lower doses are titrated to response. Reassess before repeating. |
| Severe methemoglobinemia | 15–20 mg/kg | IV | Every 6–8 h | Severe ruminant toxicity only; not the routine initial range. Administer slowly and reassess red-cell injury and methemoglobin before any repeat; higher dosing increases oxidative risk. |
| Alternative oxidant methemoglobinemia regimen | 4–15 mg/kg | IV | Every 6 h | Described with 1% medicinal solution for nitrate/nitrite/chlorate exposure. Reassess hemolysis and response before repeating; alternative to other listed ranges. |
| Historical cyanide antidote adjunct | Reference only | IV | An older adjunctive amount is 4–6 g per 454 kg body weight IV. Use only within a complete selected cyanide emergency protocol; methylene blue is not a substitute for oxygenation and an appropriate cyanide antidote. |
Horses
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Chlorate-associated methemoglobinemia | 4.4 mg/kg | IV | Once | 1% solution by IV drip; one repeat after 15–30 minutes is described if there is no response. Equine methemoglobin reduction may be poor and oxidative injury can occur. |
Indications
- Selected clinically significant methemoglobinemia
Contraindications
- Unnecessary repeated administration in patients developing oxidative hemolysis
- Renal insufficiency
- Intrathecal administration
- Methylene blue hypersensitivity
Interactions
- Serotonergic medicines may precipitate serotonin syndrome because methylene blue inhibits monoamine oxidase.
Adverse effects
- Heinz-body formation
- Hemolysis
- Injection-related reactions
Toxicity
Higher or repeated doses increase oxidative hemolysis risk, particularly in cats. Acetylcysteine is preferred first for feline acetaminophen poisoning.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Methylene blue injection 1% 1% w/v = 10 mg/mL, in the described medicinal formulation. Verify actual medicinal product; do not use laboratory dye or new methylene blue. | Injection | 10 mg / mL | IV |
Monitoring and Administration
Monitor CBC, red-cell morphology, clinical oxygen delivery and methemoglobin where available.
Administer IV slowly. For ruminants, the listed 4–10 mg/kg amount is delivered over 15 minutes; additional doses are titrated to response.
Do not confuse methylene blue with the laboratory stain new methylene blue. Medicinal strengths of 0.1% and 1% differ tenfold; verify actual bottle strength before calculating volume.
Food-animal withdrawal requires a case-specific established interval; historical values in the monograph are not adopted as current local approval.
Routine feline use is generally avoided because Heinz-body anemia can worsen oxygen delivery. Some veterinary sources nevertheless describe a single low-dose rescue treatment; such use requires an explicit benefit-risk decision and serial hematocrit.
Continue red-cell and hematocrit monitoring for at least 3 days after treatment. Extravasation or SC injection can cause tissue necrosis; avoid blue laboratory reagents as substitutes for medicinal injection. Urine may turn green-blue and interfere with urinalysis.
Historical food-animal advice differs substantially, including 14-day versus 180-day meat intervals and a 4-day milk interval. These are not adopted as current locally authorized withdrawal instructions.