Complete adverse effect profile including incidence rates and management
Important Safety Information
This is not a complete list of all possible side effects. Contact your healthcare provider if you experience any unexpected symptoms. For serious or life-threatening side effects, seek emergency medical attention immediately.
Blue-green discoloration of urine (nearly universal, >90% of patients)
Blue-green discoloration of stool (very common, >80%)
Skin discoloration — bluish tinge at injection site and diffuse skin blueing (common with IV use, >50%)
Nausea (20–30% with oral and IV use)
Vomiting (15–25% with IV administration)
Headache (15–20% of patients)
Dizziness or lightheadedness (10–15%)
Abdominal pain or cramping (10–15% with oral use)
Diaphoresis / excessive sweating (10–15% with IV use)
Transient hypertension following IV administration (10–20%)
Pain at injection site or along vein during IV infusion (common)
Restlessness or anxiety (5–10%)
Confusion or altered mental status at higher doses (5–10%)
Dysuria or urinary urgency with oral formulations (5–10%)
Mild hemolysis in patients without G6PD deficiency at standard doses (uncommon but recognized, <5%)
Serotonin syndrome — potentially life-threatening when methylene blue is combined with serotonergic agents (SSRIs, SNRIs, MAOIs, triptans, tramadol); mechanism: methylene blue is a potent MAO-A inhibitor; characterized by hyperthermia, neuromuscular abnormalities, and altered mental status
Paradoxical methemoglobinemia — at doses exceeding 7 mg/kg, methylene blue itself can oxidize hemoglobin and worsen methemoglobinemia rather than treat it
Severe hemolytic anemia — particularly in patients with G6PD deficiency who cannot generate sufficient NADPH to reduce methylene blue to leucomethylene blue; can be life-threatening
Severe hypotension with rapid IV injection — bolus administration can cause acute cardiovascular depression
Pulmonary edema — reported with high IV doses, possibly related to increased pulmonary vascular resistance
Anaphylaxis or severe hypersensitivity reactions — rare but documented; characterized by urticaria, bronchospasm, angioedema, and cardiovascular collapse
Neonatal methemoglobinemia — paradoxical worsening in neonates under 3 months due to immature reductase enzyme systems
Necrotizing enterocolitis-like injury — historical association with intra-amniotic injection during amniocentesis (use in this context is no longer performed)
Photosensitization — increased sensitivity to UV light with prolonged use or high doses
Peripheral neuropathy — reported with chronic high-dose oral use
Coronary artery spasm — isolated case reports following IV administration in susceptible patients
Methemoglobinemia in caregivers — occupational exposure through handling concentrated solutions without adequate personal protective equipment
Bladder irritation and hematuria — with chronic oral use in urinary antiseptic formulations
Rebound methemoglobinemia — recurrence several hours after initial successful treatment, especially with long-acting oxidants (dapsone)
Fetal intestinal atresia — associated specifically with intra-amniotic injection; not relevant to systemic therapeutic use
Like all medications, Methylene Blue can cause side effects. However, not everyone who takes this medication will experience them. Many side effects are dose-dependent and may improve as your body adjusts to the medication. Others may require dose adjustment or medical attention.
Contact your healthcare provider promptly if you experience:
Seek immediate emergency medical care if you experience signs of: