During the investigational use of oral N-acetylcysteine as an antidote for poisoning with acetaminophen, 11,195 cases of suspected acetaminophen overdose were reported. We describe the outcomes of 2540 patients with acetaminophen ingestions treated with a loading dose of 140 mg of oral N-acetylcysteine per kilogram of body weight, followed four hours later by 70 mg per kilogram given every four hours for an additional 17 doses. Patients were categorized for analysis on the basis of initial plasma acetaminophen concentrations and the interval between ingestion and treatment. Hepatotoxicity developed in 6.1 percent of patients at probable risk when N-acetylcysteine was started within 10 hours of acetaminophen ingestion and in 26.4 percent of such patients when therapy was begun 10 to 24 hours after ingestion. Among patients at high risk who were treated 16 to 24 hours after an acetaminophen overdose, hepatotoxicity developed in 41 percent--a rate lower than that among historical controls. When given within eight hours of acetaminophen ingestion, N-acetylcysteine was protective regardless of the initial plasma acetaminophen concentration. There was no difference in outcome whether N-acetylcysteine was started zero to four or four to eight hours after ingestion, but efficacy decreased with further delay. There were 11 deaths among the 2540 patients (0.43 percent); in the nine fatal cases in which aminotransferase was measured before treatment, values were elevated before N-acetylcysteine was started. No deaths were clearly caused by acetaminophen among patients in whom N-acetylcysteine therapy was begun within 16 hours. We conclude that N-acetylcysteine treatment should be started within eight hours of an acetaminophen overdose, but that treatment is still indicated at least as late as 24 hours after ingestion. On the basis of available data, the 72-hour regimen of oral N-acetylcysteine is as effective as the 20-hour intravenous regimen described previously, and it may be superior when treatment is delayed.

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Methaemoglobin is haemoglobin with the iron oxidised to the ferric (Fe ) state from the normal (or reduced) ferrous (Fe++) state. Methaemoglobinaemia refers to the presence of greater than the normal physiological concentration of 1 to 2% methaemoglobin in erythrocytes. Methaemoglobin is incapable of transporting oxygen. It has an intense dark blue colour; thus, clinical cyanosis becomes apparent at a concentration of about 15%. The symptoms are manifestations of hypoxaemia with increasing concentrations of methaemoglobin. Concentrations in excess of 70% are rare, but are associated with a high incidence of mortality. Methaemoglobinaemia may be congenital but is most often acquired. Congenital methaemoglobinaemia is of two types. The first is haemoglobin M disease (several variants) which is due to the presence of amino acid substitutions in either the alpha or beta chains. The second type is due to a deficiency of the NADH-dependent methaemoglobin reductase enzyme. This deficiency has an autosomal dominant transmission, and both homozygous and heterozygous forms have been reported. The heterozygous form is not normally associated with clinical cyanosis, but such individuals are more susceptible to form methaemoglobin when exposed to inducing agents. A wide variety of chemicals including several drugs, e.g. the antimalarials chloroquine and primaquine, local anaesthetics such as lignocaine, benzocaine and prilocaine, glyceryl trinitrate, sulphonamides and phenacetin, have been reported to induce methaemoglobinaemia. An intense 'chocolate brown' coloured blood and central cyanosis unresponsive to the administration of 100% oxygen suggests the diagnosis. A simple bedside test using a drop of the patient's blood on filter paper helps to confirm the clinical suspicion. Methaemoglobin can be quantitated rapidly by a spectrophotometric method.(ABSTRACT TRUNCATED AT 250 WORDS)

Four patients with elevated serum boric acid levels after single, acute ingestions of 10 to 297 grams were reported to the Rocky Mountain Poison and Drug Center (RMPDC) between January 1983 and August 1985. Systemic effects were absent. In 1983-4, 364 cases of boric acid exposure were reported to the RMPDC with only one fatality from a probable chronic ingestion. Vomiting, nausea, diarrhea, and abdominal cramps were rather common. Systemic effects were notably absent in acute ingestions. Five of three hundred sixty-four patients had measured serum levels and were the only ones hospitalized. These observations suggest that significant poisoning is unlikely to result from a single, acute ingestion of boric acid. Serum boric acid levels appear to correlate poorly with clinical toxicity following acute ingestion.

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