Nutritional supplement (NS) use is widespread in sport. This study applied an integrated social cognitive approach to examine doping attitudes, beliefs and self-reported doping use behaviour across NS users (n=96) and non-users (n=116). Following ethical approval, 212 competitive athletes (age mean=21.4, s=4.5; 137 males) completed self-reported measures of doping-related social cognitions and behaviours, presented in an online format where completion implied consent. Significantly more NS users (22.9%) reported doping compared with non-users (6.0%; U= 4628.0, p< 0.05). NS users presented significantly more positive attitudes towards doping (U= 3152.0, p<0.05) and expressed a significantly greater belief that doping is effective (U= 3152.0, p<0.05). When presented with the scenario that performance enhancing substances are effective and increase the possibility of winning, NS users were significantly more in favour of competing in situations which allow doping (U= 3504.5, p<0.05). In sum, doping use is three and a half times more prevalent in NS users compared to non-users. This finding is accompanied by significant differences in doping attitudes, norms and beliefs. Thus, this paper offers support for the gateway hypothesis; athletes who engage in legal performance enhancement practices appear to embody an 'at risk' group for transition towards doping. Education should be appropriately targeted.