Objectives: To prevent pain inhibiting their performance, many athletes ingest over-the-counter (OTC) analgesics before competing. We aimed at defining the use of analgesics and the relation between OTC analgesic use/dose and adverse events (AEs) during and after the race, a relation that has not been investigated to date. Design: Prospective (non-interventional) cohort study, using an online questionnaire. Setting: The Bonn marathon 2010. Participants: 3913 of 7048 participants in the Bonn marathon 2010 returned their questionnaires. Primary and secondary outcomes: Intensity of analgesic consumption before sports; incidence of AEs in the cohort of analgesic users as compared to non-users. Results: There was no significant difference between the premature race withdrawal rate in the analgesics cohort and the cohort who did not take analgesics ('controls'). However, race withdrawal because of gastrointestinal AEs was significantly more frequent in the analgesics cohort than in the control. Conversely, withdrawal because of muscle cramps was rare, but it was significantly more frequent in controls. The analgesics cohort had an almost 5 times higher incidence of AEs (overall risk difference of 13%). This incidence increased significantly with increasing analgesic dose. Nine respondents reported temporary hospital admittance: three for temporary kidney failure (post-ibuprofen ingestion), four with bleeds (post-aspirin ingestion) and two cardiac infarctions (post-aspirin ingestion). None of the control reported hospital admittance. Conclusions: The use of analgesics before participating in endurance sports may cause many potentially serious, unwanted AEs that increase with increasing analgesic dose. Analgesic use before endurance sports appears to pose an unrecognised medical problem as yet. If verifiable in other endurance sports, it requires the attention of physicians and regulatory authorities.