Doping in sport: performance-enhancing drugs explained

Doping in sport is again under the spotlight after fresh allegations emerged from data obtained by The Sunday Times and a German broadcaster.

Supplied by a whistleblower, the data suggests that endurance runners suspected of doping have been winning a third of the medals at Olympic Games and world championships.

Here, we take a look at the different types of performance-enhancing drugs in sport, and for what purpose they are taken.

Stimulants make people more alert and are commonly used to hide an athlete's fatigue and tiredness. This is particularly helpful after strenuous exercise when boosting energy levels can aid with the recovery process. Amphetamines are an example of banned stimulants, which is what American sprinter Justin Gatlin tested positive for back in 2006. Stimulants are addictive and linked to heart failure.

Anabolic steroids are synthetic substances that increase power by helping athletes gain muscle, allowing them to train for longer periods and at greater intensity. Tetrahydrogestrinone (THG) is the specific steroid allegedly used by many track athletes, including Marion Jones, who admitted taking THG ahead of the 2000 Olympics in Sydney. Steroids are known to raise levels of aggression and cause kidney failure.

While the structure of Diuretics vary wildly, they are used primarily to remove fluid from the body by altering sodium excretion and urine flow. This helps athletes drop in weight dramatically, most commonly ahead of an event weigh-in. The drugs can also be used in conjunction with other performance-enhancing substances, because they help to flush them out and the World Anti-Doping Agency recognise diuretics as a masking agent. Jamaican sprinter Veronica Campbell-Brown tested positive for a banned diuretic in 2013. They are said to cause severe dehydration.

Narcotic analgesics, which is less commonly used by athletes, helps athletes to perform through pain but can actually aggravate an existing injury once it has worn off. The term "analgesics" literally means "without pain" and these drugs operate as central nervous system depressants.

Peptides and hormones form another class of doping and can be split into Human growth hormone (HGH) and Erythropoietin (EPO). HGH is administered by injection and is thought to be rife among athletes desperate to build muscle and improve endurance. While there is no concrete proof that HGH improves performance, it can cause abnormal growth, heart disease, arthritis and diabetes.

EPO is a hormone naturally produced by the kidneys which can also be produced artificially and injected into the bloodstream to improve performance. It was used by Lance Armstrong throughout his cycling career, and helps to boost the production of red blood cells, increasing the amount of oxygen available to the abuser. It can cause heart problems and strokes.