The source of the medication was not specified for 3,100 (85.4%) of the NEISS-AIP cases, and the intended user was not specified for 2,982 (82.1%). On the basis of unweighted data, the most common sources of medication exposure were pills left out or pill bottles left open, which was reported in 215 (5.9%) cases. Other incidents involved medications administered in error by a parent or caregiver (3.5%) and children opening pill boxes (2.7%) or purses (3.0%). Among cases with intended users identified, the medications were intended most commonly for use by the child's grandparent (7.5%) or parent (6.6%). Exposures from OTC medications (42.2%) were slightly more common than from prescription medications (39.2%). Among the approximately 92% of cases for which the class of medication could be identified, the most common medications were central nervous system agents (e.g., acetaminophen or antidepressants) (26.9%), respiratory agents (e.g., cough and cold or anti-asthma agents) (11.6%), and musculoskeletal agents (e.g., nonsteroidal anti-inflammatory agents or muscle relaxants) (8.4%). Other common classes were cardiovascular agents (7.8%), dermatologic agents (e.g., topical antibacterial or analgesic agents) (5.3%), antihistamines (4.9%), and vitamins and therapeutic nutrients (4.5%). Prescription medications accounted for 67% of admissions to hospitals or transfers for specialized care. Among those agents specified, the most common medication classes involved in hospital admissions or transfers were anticonvulsant agents (9.6%), calcium-channel–blocking agents (6.8%), antidepressant and mood-stabilizing agents (6.2%), and oral hypoglycemic agents (6.2%).