Methylene Blue: veterinary dose and monograph

At a glance

Drug class

Veterinary drug

Also known as

Methylthioninium chloride, MB

Species with doses

Dogs, Cats, Cattle, Horses, Goats and Sheep

General dose range

1–2 mg/kg IV single dose (repeat cautiously) — check the species tables for the indication

Routes

IV

Last reviewed

1 October 2026

Indications

• Methaemoglobinaemia — nitrate/nitrite poisoning (ruminants), chlorate, oxidant drugs/toxins
• Acetaminophen-induced methaemoglobinaemia (dogs) — adjunct to N-acetylcysteine
• Surgical / diagnostic dye (e.g. parathyroid, fistula identification)

Methylene Blue dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Methaemoglobinaemia (oxidant toxicosis): Papich 5e: dogs and cats 1.5 mg/kg IV once, slowly. Plumb’s p.812: severe methaemoglobinaemia 1 mg/kg as a 1% solution slowly IV over several minutes — may repeat cautiously (Heinz-body anaemia; monitor PCV for 3 days) (Harvey 2006); phenol exposure 4 mg/kg once slowly IV with ascorbic acid 20 mg/kg PO.

Cats

• Methaemoglobinaemia (e.g. paracetamol, phenol): Papich 5e: 1.5 mg/kg IV once, slowly. Plumb’s p.812: 1–1.5 mg/kg IV one time only (Christopher 2000); 1 mg/kg as 1% slowly (Harvey). Cats readily develop Heinz-body anaemia — avoid repeating; use with acetylcysteine for paracetamol toxicity.

Cattle

• Nitrate/nitrite poisoning: Papich 5e (cattle, goats, sheep): 4–10 mg/kg IV over 15 min, repeated at lower doses to effect or every 6–8 h; 15–20 mg/kg for severe toxicity. Plumb’s p.812: 4–15 mg/kg of 1% solution IV q6h (Osweiler); nitrate 5–15 mg/kg (Hall). FARAD: milk ≥4 days; meat 14 days considered sufficient (an extremely conservative 180 days has been suggested for carcinogenicity concerns).

Horses

• Methaemoglobinaemia (chlorate toxicosis): Plumb’s p.812 (Schmitz 2004): 4.4 mg/kg of 1% solution by IV drip, repeatable in 15–30 min if no response. Horses with red maple toxicosis respond poorly — supportive care is central.

Goats

• Nitrate/nitrite poisoning / methaemoglobinaemia: Papich 5e: cattle, goats and sheep 4–10 mg/kg IV over 15 min, repeated to effect (15–20 mg/kg for severe cases). FARAD (Plumb’s): milk ≥4 days, meat 14 days.

Sheep

• Nitrate/nitrite poisoning / methaemoglobinaemia: Papich 5e: cattle, goats and sheep 4–10 mg/kg IV over 15 min, repeated to effect (15–20 mg/kg for severe cases). FARAD (Plumb’s): milk ≥4 days, meat 14 days.

Buffalo

No buffalo-specific doses are on file, so buffalo are dosed as cattle.

• Nitrate/nitrite poisoning: Papich 5e (cattle, goats, sheep): 4–10 mg/kg IV over 15 min, repeated at lower doses to effect or every 6–8 h; 15–20 mg/kg for severe toxicity. Plumb’s p.812: 4–15 mg/kg of 1% solution IV q6h (Osweiler); nitrate 5–15 mg/kg (Hall). FARAD: milk ≥4 days; meat 14 days considered sufficient (an extremely conservative 180 days has been suggested for carcinogenicity concerns).

Contraindications

Cats — avoid or use only minimal single doses (Heinz-body haemolytic anaemia). G6PD-type oxidative susceptibility. Significant renal impairment (renally excreted; blue discolouration). Repeated/high dosing (paradoxical methaemoglobinaemia and haemolysis). Concurrent serotonergic drugs (MB is a potent MAO inhibitor — serotonin syndrome). Known hypersensitivity.

Species-specific warnings

Cats: Cats are highly prone to methylene-blue-induced Heinz-body haemolytic anaemia — avoid, or use only a single minimal dose when no alternative exists.

Cattle: Ruminants tolerate higher doses for nitrate poisoning; still give slowly IV and monitor for haemolysis.

Adverse effects

• Heinz-body haemolytic anaemia (oxidant effect — cats especially)
• Blue-green discolouration of urine, tissues, skin (harmless but alarming)
• Paradoxical methaemoglobinaemia at high/repeated doses
• Serotonin syndrome with serotonergic drugs (MAOI activity)
• Local irritation/necrosis if extravasated

Drug interactions

• Serotonergic drugs (SSRIs, tramadol, MAOIs): serotonin syndrome (methylene blue is a potent MAO inhibitor) — avoid combining
• Oxidant drugs: additive Heinz-body/haemolysis risk
• No significant beneficial drug pairings beyond NAC for acetaminophen

Pregnancy and lactation

Avoid in pregnancy where possible (oxidant/teratogenicity concerns); use only for serious methaemoglobinaemia with no alternative.

Monitoring

• Methaemoglobin level / mucous-membrane colour (response)
• PCV and red-cell morphology (Heinz bodies/haemolysis) for 2–3 days after
• Renal function (renally excreted)
• Avoid repeat dosing, especially in cats/dogs

Toxicity and overdose

The cardinal toxicity is oxidative haemolysis (Heinz-body anaemia) — markedly in cats, and with high or repeated dosing in any species — plus paradoxical methaemoglobinaemia above therapeutic doses. Serotonin syndrome can occur with serotonergic co-medication. Tissue/urine turn blue-green (cosmetic).

Management: Stop the drug. Supportive care for haemolytic anaemia (oxygen, IV fluids, transfusion if severe) and for paradoxical methaemoglobinaemia (ascorbic acid; in severe cases exchange transfusion). Manage serotonin syndrome with cyproheptadine/cooling. There is no antidote to methylene blue itself.

Clinical pearls

• The antidote that "un-browns the blood" — converts methaemoglobin back to functional haemoglobin in nitrate/oxidant poisoning
• A double-edged sword: too much (or any in cats) causes oxidative Heinz-body haemolysis — dose low, give once, and largely AVOID in cats
• For acetaminophen-toxic dogs, pair a single conservative MB dose with N-acetylcysteine (the primary antidote)
• It is a potent MAO inhibitor — do not combine with serotonergic drugs (serotonin syndrome)
• Warn owners that urine/tissues turn blue-green — harmless

Mechanism of action

At therapeutic (low) doses methylene blue is reduced by NADPH-methaemoglobin reductase to leucomethylene blue, which non-enzymatically reduces ferric (Fe3+) methaemoglobin back to functional ferrous (Fe2+) haemoglobin — restoring oxygen-carrying capacity. At HIGH doses (or in cells lacking adequate NADPH, e.g. feline red cells) it acts as an oxidant itself, generating methaemoglobin and Heinz bodies — the dose-dependent paradox that limits its use.

Formulations and products

• Injection 10 mg/mL (1 %)
• Injection 5 mg/mL (0.5 %)
• Methylene Blue injection — 10 mg/mL (1 %) / 5 mg/mL, Injection (human-label)

Storage

Injection: 15–30 °C, protect from light; use a dilute solution and give slowly IV (avoid extravasation).

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other hormones, emergency drugs, supplements and other drugs

• Metformin
• Methimazole
• Methocarbamol
• Methylcobalamin (Active B12)
• Mitotane (o,p'-DDD)
• Mycophenolate Mofetil
• N-Acetylcysteine (NAC)
• Naloxone HCl

Frequently asked questions