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Background Little is known about the relation of adverse drug reactions (ADRs) to self-use of medications. Objective The aim of this study was to determine the frequency and severity of ADRs related to self-medication (ADR-SM) among emergency department (ED) patients and to describe their main characteristics. Methods A prospective, cross-sectional, observational study was conducted over a period of 8 weeks (1 March to 20 April 2010), in the ED of 11 French academic hospitals. Adult patients presenting to the ED during randomization periods were included, with the exception of cases of self-drug poisoning, inability to complete self-medication questionnaire, or refusal. Clinical outcomes were assessed as well as history of self-medication behaviours and all drugs taken. All doubtful files and those related to ADR-SM were systematically reviewed by an expert committee. Results A total of 3,027 of 4,661 patients presenting to the ED met the inclusion criteria. Of these, 84.4 % declared a self-medication behaviour, 63.7 % took at least one non-prescribed drug during the previous 2 weeks and 59.9 % took a prescribed medication. A total of 296 patients experienced an ADR (9.78 %), of which 52 (1.72 %) were related to self-medication. Those ADRs related to self-medication included prescribed drugs (n = 19), non-prescribed drugs (n = 17), treatment discontinuation (n = 14), and interactions between non-prescribed and prescribed drugs (n = 2). The ADRs attributed to non-prescribed drugs represented 1 % of all patients taking non-prescribed drugs (n = 1,927). ADR severity was significantly lower for those related to self-medication (p = .032). Conclusion Self-medication is frequent; its potential toxicity should not be neglected, taking into account the rate of adverse drug reactions in about 1 % of ED patient. Electronic supplementary material The online version of this article (doi:10.1007/s40264-013-0114-y) contains supplementary material, which is available to authorized users.