Urea herbicides, which act by inhibiting photosynthesis, were introduced in 1952 and are now used as pre- and post-emergence herbicides for general weed control in agricultural and non-agricultural practices. Urea herbicides are generally of low acute toxicity and severe poisoning is only likely following ingestion when nausea, vomiting, diarrhoea and abdominal pain may occur. As urea herbicides are metabolised to aniline derivatives, which are potent oxidants of haemoglobin, methaemoglobinaemia (18-80%) has been documented, as well as haemolysis. Treatment is supportive and symptomatic. Methylthioninium chloride (methylene blue) 1-2mg (the dose depending on the severity of features) should be administered intravenously over 5-10 minutes if there are symptoms consistent with methaemoglobinaemia and/or a methaemoglobin concentration >30%.