The potential for supplement use to result in doping infringements is likely to be of concern for anyone involved in sports nutrition. The available data indicates that between 40–70% of athletes use supplements, and that between 10–15% of supplements may contain prohibited substances. Such data indicates that there is a considerable risk of accidental or inadvertent doping through using supplements. Accordingly, this paper sets out to provide an overview of the currently available empirical evidence of accidental doping by supplement use. In carrying out this task, the authors refer to press releases and proxy measures associated with nutritional supplement use, as well as statistical data on supplement contamination rates and doping infractions. A number of different indications as to the percentage of doping cases that might be attributed to supplement use are presented, ranging from 6.4% to 8.8%. Such percentages are not comparable; instead they are provided as indications as to how difficult it is to ascertain or estimate the scale of this problem. Although some forms of estimation can be made, it is suggested that it is currently not possible to quantify the scale of the problem. By way of conclusion, it is argued that antidoping regulators may wish to review current data gathering and information provision systems so that the problem of inadvertent doping can be more directly assessed as a factor in sports doping overall.