Methylene Blue

What it is used for

• For methemoglobin-producing toxins (nitrites, nitrates, chlorates)
• For cyanide toxicity
• For methemoglobin-producing toxins (nitrites, nitrates, chlorates)
• For cyanide toxicity
• Methemoglobinemia secondary to phenol exposure
• Methemoglobinemia
• Severe methemoglobinemia
• For methemoglobin-producing toxins (nitrites, nitrates, chlorates)
• For nitrate poisoning
• For cyanide toxicity
• Methemoglobinemia secondary to chlorate toxicity
• To preferentially stain islet-cell tumors of the pancreas
• Methemoglobinemia secondary to phenol exposure

How it is given

Possible side effects

• Heinz body anemia
• Red cell morphological changes
• Methemoglobinemia (at high doses)
• Decreased red blood cell life span
• Necrotic abscesses (if injected SC or extravasated IV)
• Green-blue discoloration of urine
• Heinz body anemia (especially in cats)
• Paradoxical methemoglobinemia (at high doses)
• Tissue necrosis (if extravasated)
• Blue/green discoloration of urine, feces, and mucous membranes
• Nausea and vomiting

Cautions & contraindications

• Cats (considered contraindicated by most clinicians due to severe sensitivity)
• Renal insufficiency
• Hypersensitivity to methylene blue
• Intraspinal (intrathecal) injection
• Lactating dairy animals
• Severe renal impairment
• Patients with G6PD deficiency (can cause severe hemolysis)
• Use with extreme caution in cats due to high susceptibility to Heinz body formation
• > **WARNING:** Cats are highly sensitive to methylene blue and can rapidly develop **Heinz body anemia** and methemoglobinemia. Dogs and horses can also develop these effects at relatively low dosages. * **Administration:** Must be given IV. Extravasation or subcutaneous injection can cause severe necrotic abscesses. * **Efficacy:** Considered relatively ineffective in reducing methemoglobin in horses. * **Food Animal Residues:** FARAD recommends a minimum milk withdrawal time of 4 days. Meat withdrawal is conservatively recommended at 180 days due to carcinogenicity concerns, though some data suggest 14 days may be sufficient. * **Pregnancy:** FDA Category C. Safe use during pregnancy has not been demonstrated.

Drug interactions

• SSRIs (e.g., Fluoxetine): Methylene blue is a potent reversible monoamine oxidase inhibitor (MAOI); concurrent use can precipitate fatal serotonin syndrome.
• Tricyclic Antidepressants (TCAs): Increased risk of serotonin toxicity.

Frequently asked questions