Adverse reactions to iodine containing contrast media (CM) have been described after endoscopic retrograde cholangiopancreaticography (ERCP) but have been difficult to study due to their extremely low incidence. Adverse reactions can be classified by severity (mild, moderate or severe) or underlying mechanism (idiosyncratic versus nonidiosyncratic). Nonidiosyncratic reactions are related to the dose and osmolality of the CM; these are not observed during ERCP due to the relative small amount of systemic absorption of CM. Idiosyncratic reactions on the other hand are not dose-dependent and generally occur shortly after administration of CM.