The increasing availability of food supplements, aggressive media advertising, and common beliefs that these substances have only positive effects on health and sport performance indicate a need for continuous monitoring of this phenomenon. The aim of this study was to investigate the habits and beliefs related to diet supplementation among medical, health professional, and other university/high school students by means of a cross-sectional anonymous survey online. Among the respondents aware of supplements, 37.4% were taking or had taken them in the past. Food supplement use was more common among university students (in particular, those in health professional graduate courses) than high school students. Individual sport practice, rather than team sport, was associated with higher likelihood of food supplement use. Multivitamins were most commonly used, while weight-loss formulations were the least popular. Strikingly, filling nutrient gaps was statistically not considered the main reason for taking food supplements. Instead, they were used to enhance mental performance or enhance well-being. There was statistical evidence that students not enrolled in health or medical professional studies strongly agreed more often than medical students that taking food supplements prevents illness. These results indicate a striking difference between the evidence-based and personal reasons for food supplement use. Arguably, it calls for an improvement in education about diet supplementation and a change in attitude of health care providers to its implementation.