Depends on the athletes physiological state (health and potential contraindications) and what he/she decides to use, but in general health and fitness supplements are safe. Prescription or over-the-counter drug (OTC) diet pills are something that should be discussed with a doctor and many of these types of diet medications are outlawed in professional or Olympic sports.
If you are worried about the safety of common supplements used by athletes, consider this: Maximizing potential during high-level competition involves athletes exploiting all available "natural" resources. In the 2008 Olympics, 90% of the 11,000 athletes reported regularly using dietary supplements. There was not a single supplement contamination or sickness case, giving a “thumbs up” to the dietary supplement industry for making safe, unadulterated substances. Additionally there is unequivocal evidence that a limited number of natural substances prepared and ingested properly can safely improve training-induced size or performance for many athletes. Historically, however, athletes have had a tendency to not follow directions. Many subscribe to the old adage, “if a little works, more is better.” The practice of overconsumption of anything—such as foods, dietary compounds, and drugs—can lead to problems. On the other hand, proper supplementation for performance has often been shown to generate truly remarkable benefits, and this in itself can save many athletes from turning to illegal anabolic steroid use, which has well-known harmful side effects. The rationale behind using nutritional strategies, including supplements, is to avoid training plateaus. Therefore, when the benefits of training and diet on muscle mass and performance have stabilized, specific nutrient supplement regimens may play a role in plateau avoidance and progressive development for many athletes.
Below is what most competitive athletes use and is perfectly safe when manufactured by a reputable company with a history in sports and fitness:
Active multivitamin mineral formula
• Take 1 daily with breakfast or any other larger meal
• Use SuperCalcium+ and SuperOmega-3 as necessary – i.e. not getting enough from food
dotFIT Pre/Post Meal Replacement & Workout Formula
• Take 2 scoops 10-40 minutes before workout or use favorite dotFIT® bar for convenience
• Take 2 scoops (or dotFIT bar) immediately post workout