OBJECTIVES: To estimate hospitalization costs attributable to obesity (CAO) in patients with adverse drug events (ADE) in the US hospital. METHODS: The study used the Nationwide Inpatient Sample (NIS) of the Healthcare Cost and Utilization Project (HCUP) from 2001 to 2009. The NIS includes sampling weight and design variable for generating national representative estimates.  ADE were defined as poisoning due to inappropriate uses or medication errors, and an adverse drug reaction. The cost to charge ratios were applied to estimate hospitalization costs using the hospital charge. The recycled prediction method was employed to estimate CAO.  This method estimated parameters by running a generalized linear model with log link function and gamma distribution of hospital costs on all the study variables. CAO was defines as the differences between the predicted mean hospitalization costs for obese patients with ADE and the predicted mean hospitalized costs if obese patients with ADE could be a normal weight patients with ADE.  All costs were converted to 2010 US dollars. RESULTS: A total of 2,177,755 admissions due to ADE were identified.  Among these admissions, 4.3% of them were obese patients.  Obese patients were older than non-obese patients (46.6 vs. 43.7, p<0.001) and stayed longer in hospital than non-obese patients (3.4 vs. 2.9 days, respectively).  The average hospitalization costs were $9099 (95%CI:$8,926-$9272) for obese and $8014 (95%CI:$7978-$8051) for non-obese patients.  Overall, the average CAO in patients with ADE was predicted to be $725 (95%CI:$720-$729) after adjusting for patients characteristics and comorbidities.  CAO were increased as patients age increased from $457 (95%CI:$446-$468) for age 0-17 years old to $1,051($993-$1,110) for age ≥85years old.  CONCLUSIONS: Obese patients with ADE have significantly higher hospitalization costs compared to non-obese patients with ADE.  Since the additional costs due to obesity are avoidable, the additional efforts to lose weight are strongly recommended.