Serum acetaminophen concentrations after acute overdose are not altered by opioid co-ingestion

Serum acetaminophen concentrations are of critical importance in determining the need for acetylcysteine therapy after acute acetaminophen overdose. Limited data suggest opioid co-ingestion might alter acetaminophen pharmacokinetics. The present study was designed to examine serum acetaminophen concentrations after acute overdose, and to compare between patients that co-ingested an opioid and those that did not. A prospective study of consecutive patients that presented to hospital within 16 hr of acute acetaminophen overdose. Equivalent 4-hr acetaminophen concentrations were calculated using the serum acetaminophen concentration at a fixed interval 3 to 16 hr after ingestion. Groups were compared using Mann Whitney tests. There were 990 patients; 295 (29.8%) had co-ingested an opioid, and 695 had not. The median (interquartile range) stated dose was 10 g (6-16 g) vs. 10 g (7-16 g) respectively (P = 0.94), interval between ingestion and acetaminophen determination was 4.5 hr (4.0-6.0 hr) vs. 4.5 hr (4.0-5.5 hr) respectively (P = 0.41), and serum acetaminophen concentration was 56 mg/l (24-105 mg/l) va. 60 mg/l (23-129 mg/l) respectively (P = 0.25). A positive relationship was noted between stated dose and equivalent 4-hr serum acetaminophen concentration, but did not differ between groups. The acetaminophen dose-concentration relationship was similar in patients that did and did not co-ingest an opioid. Therefore, early serum acetaminophen concentrations can be used to determine the extent of drug exposure, irrespective of whether an opioid has been co-ingested.