N-Acetylcysteine (NAC) dosing for acetaminophen (APAP) overdose is weight based (150 mg/kg intravenous or 140-mg/kg oral loading dose) and, in the United States, the dosing protocol recommends using a maximum patient weight of 100 and 110 kg, respectively. Little clinical data describe the use of NAC for APAP poisoning in patients weighing >100 kg. The aim of this study was to describe the demographics, outcomes, and adverse event (AE) rates of patients weighing >100 kg treated with oral or IV NAC for APAP poisoning. Patients were identified from a multicenter retrospective NAC safety study for APAP overdose. We included patients with a recorded weight. Trained chart abstractors used a standardized form. Selected data included age, gender, weight, serum alanine transaminase, and aspartate transaminases, coingestants, NAC administration route, ingestion type, AEs, and outcome [hepatotoxicity (alanine transaminase > 1000 U/L), liver transplant, or death]. Descriptive statistics were used. Of 503 study patients, 37 (7.4%) had recorded weights >100 kg. The median (range) weight was 110 kg (101-160). The median (range) dosing for patients treated with oral NAC was 140 mg/kg (127-143 mg/kg) and 150 (108-168) mg/kg for IV NAC. Hepatotoxicity occurred in 12/36 (33.3%) patients. Death occurred in 4/36 (11.1%) patients. Thirteen NAC-related AEs occurred in 8 patients (1.6 per person). All AEs were related to NAC and were rated nonserious by the reviewer. Clinicians use an actual weight-based NAC dose rather than a maximum weight cutoff dose. Hepatotoxicity was common in our cohort. AEs were relatively common but not serious.