Using a national surveillance system to identify adverse events A(Es) involving common oral over-the-counter (OTC) cough and cold medications, an expert panel evaluated and assigned causal relationships between AEs and active ingredients in the medications. Of the 4,756 adverse events identified, 10.8% were due to a medication error; nearly all of these errors (93.2%) were attributed to the wrong dose of medication. The most common medication errors involved diphenhydramine and dextromethorphan. Almost half of medication errors (45.8%) involved children between the ages of 2 and 5 years old and involved administration by either a parent (45%) or alternative caregiver (28.8%). Continued standardization of medication measuring devices, concentrations and units, as well as consumer education, is needed to further decrease medication errors from these common OTC medications.