What do you mean by harmful …

Some ergogenic aids are known to have harmful side effects and this is the reason why they are banned by sports governing authorities because they are unsafe and unethical. 

The most abused aids include the following:

• Anabolic steroids: These are synthetic hormones used to increase muscle mass and strength. They also produce a “steroid rush”, a state of euphoria and decreased fatigue that allows the athlete to train harder and longer. Many adverse effects have been documented from using these steroids
• Blood doping: This is another dangerous ergogenic aid. It involves taking blood or blood products such as erythropoietin (EPO), a hormone that stimulates the bones to make red blood cells, to an athlete to improve endurance and speed. It can also have harmful side effects
• Human growth hormone (HGH): HGH is a widely abused ergogenic aid by body builders