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Methylene blue
Thiazine dye (methaemoglobinaemia antidote; formerly an oral urinary medicine). Used in veterinary practice (cat); overdose thresholds from published sources, not treatment doses.
Overdose
Used in veterinary practice at veterinary doses; human tablets and gels are much stronger and cannot be dosed accurately for an animal. The table shows where harm has been described after a single ingestion, mg per kg of body weight: overdose thresholds, not treatment doses.
| Species | Signs begin | Severe | Deaths reported | Source |
|---|---|---|---|---|
| Cat | not established | not established | not established |
FDA 21 CFR 500.27 · Schechter et al. 1973 +1
High confidence
Clinical reports, a US regulation and a controlled study in 40 cats (Schechter et al. 1973; 21 CFR 500.27; Rumbeiha & Oehme 1992). Guideline or consensus statement Recommendations agreed by a panel of veterinary specialists after reviewing the evidence. They guide practice and do not authorise a product. Code of Federal Regulations, Title 21, 21 CFR 500.27 oral methylene blue urinary antiseptics: Heinz-body haemolytic anaemia in cats on label use, can be fatal Scientific article A study or review published in a scientific journal. Its findings apply to the animals and conditions it studied. Journal of the American Veterinary Medical Association, J Am Vet Med Assoc 1973;162(1):37-44, 1973 Heinz-body haemolytic anaemia in cats with methylene blue urinary antiseptics Scientific article A study or review published in a scientific journal. Its findings apply to the animals and conditions it studied. Veterinary and Human Toxicology, Vet Hum Toxicol 1992;34(2):120-122, 1992 one 1.5 mg/kg IV dose reverses methaemoglobinaemia without Heinz bodies; two doses raise them |
Cat
Signs: Heinz bodies, haemolytic anaemia; can be fatal.
Given by mouth for days, as in old urinary antiseptic tablets, methylene blue caused Heinz-body haemolytic anaemia in cats, sometimes fatal; the US FDA regards oral methylene blue products for cats as unsafe. As the antidote to methaemoglobinaemia one intravenous dose of 1.5 mg/kg reversed it in cats without anaemia; a second dose 4 hours later raised Heinz bodies.
Safety information
Known Contraindications
-
Warning
(Cat) Methylene blue in cats: not by mouth as a urinary antiseptic (Heinz-body haemolytic anaemia, sometimes fatal). As the antidote to methaemoglobinaemia give a single intravenous dose (1.5 mg/kg was safe); a repeat dose raises Heinz bodies, so check the blood before repeating.Sources: FDA
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