Acetaminophen (APAP) protein adducts are a biomarker of APAP metabolism, reflecting oxidation of APAP and generation of the reactive metabolite N-acetyl-p-benzoquinone imine. High levels of adducts correspond to liver toxicity in patients with APAP-related acute liver failure. Adduct formation following low-dose exposure to APAP has not been well studied. APAP protein adducts were measured in blood samples collected from fasted individuals who participated in a crossover study of APAP (80 mg/kg) comparing extended release (ER) and immediate release (IR) formulations. METHODS: Adducts were quantified in all postdose blood samples using a validated high-performance liquid chromatography electrochemical detection (HPLC-EC) assay. RESULTS: Comparison of pharmacokinetic parameters for adducts did not reveal significant differences between ER and IR formulations, with one exception. Formation rates for adducts were faster for IR than the ER formulation (0.420a??A?a??0.157 vs. 0.203a??A?a??0.080 1/h), respectively. Maximum plasma concentrations (C(max)) of adducts for IR and ER were 0.108 (A?0.020) and 0.100 (A?0.028) nmol/ml serum, respectively, and were two orders of magnitude lower than adduct levels previously reported in adults with acute liver failure secondary to APAP. CONCLUSIONS: APAP protein adducts are rapidly formed following nontoxic ingestion of APAP at levels significantly lower than those associated with acute liver failure.