What is doping?
Doping means athletes taking illegal substances to improve their performances.
There are five classes of banned drugs, the most common of which are stimulants and hormones. There are health risks involved in taking them and they are banned by sports' governing bodies.
According to the UK Anti-Doping Agency, substances and methods are banned when they meet at least two of the three following criteria: they enhance performance, pose a threat to athlete health, or violate the spirit of sport.
Why is it an issue now? A brief history of doping
The use of stimulants and strength-building substances in sport is held to date back as far as Ancient Greece, but it was during the 1920s that restrictions about drug use in sport were first thought necessary. In 1928 the International Association of Athletics Federations (IAAF) - athletics' world governing body - became the first international sports federation to ban doping.
In 1966, the world governing bodies for cycling and football were the first to introduce doping tests in their respective world championships, with the first Olympic testing coming in 1968, at the Winter Games in Grenoble and Summer Games in Mexico. By the 1970s, most international federations had introduced drug-testing.
A major drug scandal at the 1998 Tour de France underlined the need for an independent international agency to set standards in anti-doping work. The World Anti-Doping Agency (Wada) was established the following year.
In January 2013, the retired American cyclist Lance Armstrong admitted to doping in an interview with Oprah Winfrey, and was stripped of his seven Tour de France wins and banned from sport for life.
In December last year, a German TV documentary alleged as many as 99% of Russian athletes were guilty of doping, although the Russian Athletics Federation described the allegations as "lies".
Since then, there have been numerous further allegations of doping in athletics.
Prior to Armstrong's confession, Ben Johnson was probably the world's highest-profile drugs cheat. The Canadian sprinter tested positive for anabolic steroids at the 1988 Olympic Games in Seoul.
Johnson had won the 100m in a world record of 9.79 seconds but was stripped of his gold medal after the positive test and sent home in disgrace.
British sprinter Dwain Chambers was banned from competition for two years in 2004 after being found guilty of taking the anabolic steroid THG, while compatriot Linford Christie, a former sprint champion, was suspended from athletics in 1999 after failing a drugs test.