The use of legal ergogenic aids is widespread in athletic populations and varies between 40% and 100% of athletes of both sexes, depending on the sport discipline and the level of competition. Often, athletes take simultaneously various products without regard to optimal dose schemes and total dosage of some ingredients or synergic and antagonistic interactions between them. Thus, some commonly used nutritional aids may lead to health disorders. The legal nutritional supplements which are permitted by WADA and that are supported by reliable evidence of promoting athletes’ physical performance include caffeine, creatine, carbohydrate drinks/gels/bars, β-alanine, bicarbonate, nitrate (beetroot juice) and proteins. Caffeine use shows the most side effects and these become more common with caffeine doses over 9 mg/kg of body mass. Overdose may lead to cardiotoxicity with significant cardiovascular side effects such as tachycardia, coronary and peripheral vasoconstriction, and elevated blood pressure especially in caffeine-naive recreational athletes. More research studies on biomedical side effects and educational campaigns particularly aiming at children and athletes of developmental ages can have a significant influence on nutritional supplements use in sports and may act as the most powerful tools for an effective fight against the indiscriminate use of legal ergogenic aids.