Letter: Prevention of acetaminophen (paracetamol) poisoning

Acetaminophen and its toxicity

Acetaminophen (APAP) is considered one of the safest of all minor analgesics, but when taken in large doses (greater than 10 g) toxicity occurs. Severely poisoned patients experience hepatic and/or renal failure. The major metabolic pathway of APAP is formation of glucuronide and sulfate conjugates. A minor pathway is formation of a reactive metabolite that conjugates with glutathione (GSH). When GSH is depleted, the reactive metabolite causes necrosis of hepatic and other tissues. Treatment of APAP toxicity involves supplying alternate sulfhydryl donors or inhibiting oxidative formation of the reactive metabolite. Estimation of plasma APAP levels is necessary for effective treatment.

The use of methionine for acute paracetamol poisoning

Since September 1974 the London Centre of the National Poisons Information Service has advocated the use of methionine for acute paracetamol poisoning and this paper presents a preliminary report of its outcome.

Seventeen patients, all of whom had high plasma paracetamol levels, received oral methionine within ten hours of the ingestion of the overdose. Eleven of these patients suffered no hepatic damage, while the remaining six showed varying degrees of liver failure with a mean maximum aspartate aminotransferase (AST) of 1,098 iu/l. There were no deaths in this group. Among fourteen patients who had taken similar paracetamol overdoses and received no antidotal treatment, all developed liver damage and seven of these died. These two groups were not, however, accurately matched for dose of paracetamol, plasma paracetamol levels or interval before reaching hospital.

Comparison was therefore made with two other recently published series of untreated patients. A significant reduction was noted in the incidence of liver damage and in the mean maximum AST.

Although this survey was not controlled, it is suggested that oral methionine, which has a low incidence of side-effects, may be effective in reducing the incidence and severity of paracetamol-induced liver damage, provided the overdose is not massive.

A theoretical approach to the management of paracetamol overdosage

Possible ways of treating paracetamol hepatotoxicity are briefly reviewed. This toxicity appears to be due to an oxidation metabolite which is normally detoxified by conjugation with glutathione, but when in excess, as in paracetamol overdosage, binds covalently to hepatic proteins to cause liver necrosis. Depletion of hepatic glutathione may also lead to other contributory disorders of cellular metabolism. Physical methods of preventing this would appear to be of little value, and treatments are being developed which aim to lessen the production of the toxic metabolite, antagonize its attachment to hepatic tissue or to combat this toxicity via basic cellular mechanisms. At this moment the most effective substances appear to be glutathione precursors and other compounds which interfere with oxidation of paracetamol.