Methylene Blue

What is this medication for?

• Methemoglobinemia from oxidative toxins (acetaminophen, phenol, nitrates/chlorates); cyanide toxicity adjunct
• Intraoperative staining of pancreatic islet-cell tumors
• Methemoglobinemia — last-resort single low dose only (most clinicians contraindicate)

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Drug-induced methemoglobinemia (Overdose paradox)
• Urine/secretion staining (blue-green) (Expected)

Worth a phone call to your veterinarian

• Heinz body low red blood cell counts, RBC morphologic changes, ↓ RBC lifespan (The dose-limiting toxicity)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Cats (consensus of most clinicians); renal insufficiency; hypersensitivity; intrathecal route; lactating dairy animals
• has or has had: Repeat dosing escalates Heinz risk — check hematocrit for 3 days post-treatment; relatively ineffective in horses

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Serotonergic drugs (SSRIs etc. — human data)
  • Oxidizing agents

Storage and handling

1% solution (10 mg/mL). Never intrathecal. Avoid under the skin/perivascular extravasation (necrotic abscesses).

Good to know

• The antidote that can cause the disease: low dose reduces metHb, high dose oxidizes Hb — respect the 1 mg/kg canine / 1–1.5 mg/kg single-dose feline line.
• In feline acetaminophen toxicity, NAC + ascorbic acid carry the methemoglobinemia load; methylene blue is a desperate single-dose measure because cats' vulnerable hemoglobin makes the cure nearly as dangerous as the poison (Heinz bodies).
• Intraoperative insulinoma localization is the quirky second life of this dye (stains islet-cell tumors reddish-violet against dusky-blue pancreas).