Here's a look at the use of performance-enhancing drugs in sports.
"Doping" by professional athletes has been acknowledged as a problem since at least the 1960s.
The issue gained prominence as a result of the Bay Area Laboratory Co-Operative (BALCO) investigation, tell-alls by former professional athletes, a 2007 report on drugs in professional baseball and a scandal involving Russia's 2014 Olympic team.
About Performance-Enhancing Drugs:
There are several types of performance-enhancing drugs, including anabolic steroids, stimulants, human growth hormone and diuretics.
Anabolic steroids are natural and synthetic substances which help build muscle mass, enabling athletes to train harder and recover quickly from strenuous workouts.
Tetrahydrogestrinone, also known as THG or the Clear, is a powerful steroid purportedly used by such high profile athletes as track star Marion Jones and baseball player Barry Bonds.
Stimulants, including amphetamines, impact the central nervous system, increasing alertness and decreasing appetite.
Human growth hormone (HGH) is taken for improved endurance and strength.
Androstenedione is a supplement that was sold over-the-counter until the FDA took action in 2004. It is banned by the NFL, Olympics, NCAA and MLB. The supplement is an anabolic steroid precursor, meaning that the body converts it into testosterone.
1967 - The International Olympic Committee (IOC) establishes a Medical Commission in response to an increase in the usage of performance enhancing substances.
1981 - After American discus thrower Ben Plucknett tests positive for steroids, he is banned from participating in future events by the International Amateur Athletics Federation (IAAF) and he is stripped of his world record.
1987 - The National Football League (NFL) begins testing players for steroids.
1988 - Congress passes the Anti-Drug Abuse Act, which makes possession and distribution of anabolic steroids for non-medical purposes a crime.
1990 - Congress strengthens the 1988 law by classifying anabolic steroids as a controlled substance.
2002 - Federal authorities launch an investigation into BALCO, a California lab that is suspected of selling performance enhancing drugs to elite athletes.
2003 - Major League Baseball begins testing players for steroids.
February 2005 - "Juiced: Wild Times, Rampant 'Roids, Smash Hits and How Baseball Got Big," an autobiography by retired baseball star Jose Canseco, is published. In the book, Canseco recounts his own steroid use and implicates other players.
March 2005 - Six former and current Major League Baseball players testify before the House Government Reform Committee about drugs in baseball. They include Mark McGwire, Sammy Sosa and Canseco.
March 2006 - MLB Commissioner Bud Selig announces an investigation of steroid use among pro baseball players. Former US Sen. George Mitchell will lead the investigation.
August 22, 2006 - The USADA bans sprinter Justin Gatlin for eight years after he tests positive for banned substances a second time. Gatlin is also forced to forfeit his 100-meter world record.
May 2007 - 1996 Tour de France winner Bjarne Riis admits using performance-enhancing drugs to win his title. Race organizers tell him to return his yellow first-place jersey.
September 20, 2007 - Cyclist Floyd Landis is stripped of his 2006 Tour de France title and he is banned for two years after a positive test for synthetic testosterone.
February 2008 - Former New York Mets clubhouse employee Kirk Radomski is sentenced to five years probation after pleading guilty to distributing steroids.
February 2009 - Alex Rodriguez admits to using performance-enhancing drugs while playing for the Texas Rangers.
January 2010 - McGwire admits to using steroids during his career.
February 2012 - Three-time Tour de France winner Alberto Contador is stripped of his 2010 title for doping.
June 2012 - The USADA confirms that it is opening proceedings against Lance Armstrong and five former teammates. Armstrong denies the charges. (For more details about Armstrong's case, see our Lance Armstrong Fast Facts).
August 2012 - American cyclist Tyler Hamilton is stripped of his gold medal from the 2004 Olympics after he admits to doping.
January 2013 - MLB announces it will begin random testing for HGH.
August 2014 - Anthony Bosch, the founder of a Miami anti-aging clinic, surrenders to the Drug Enforcement Administration. He later pleads guilty to a charge of distributing steroids to athletes. His sentence is four years in federal prison.
September 2014 - The NFL and NFL Players Association reach an agreement regarding the league's performance-enhancing drug policy. The agreement calls for HGH testing and an overhaul of the drug program.
January 2015 - Kenya's Rita Jeptoo, a three-time Boston Marathon champion, is banned from competition for two years for doping.
September 2015 - The DEA announces that 90 people have been arrested and 16 underground steroid labs have been shut down in a sweeping drug bust called Operation Cyber Juice.
November 9, 2015 - A WADA report details evidence of doping in Russian athletics and a "deeply rooted culture of cheating at all levels." Russia is later provisionally suspended as a member of the International Association of Athletics Federations.
March 2016 - At a press conference, tennis player Maria Sharapova admits to failing a drug test at the Australian Open. She is initially suspended for two years, but the ban is later reduced to 15 months.
July 18, 2016 - A WADA report alleges Russia ran a state-sponsored doping program during the 2014 Sochi Winter Olympics. On December 9, 2016, WADA releases an update to the report concluding that a"systematic and centralized cover-up" benefited more than 1,000 Russian athletes across 30 sports.
August 4, 2016 - The IOC announces that 271 athletes from the 389-member Russian Olympic team have been cleared to participate in the 2016 Rio Olympics. The rest of the team - 118 athletes - are banned in the wake of the doping scandal.
August 11, 2016 - John Anzrah, a sprint coach for the Kenyan Olympic team, is sent home after allegedly posing as an athlete to take a drug test. He is the second Kenyan running coach to face allegations that he tried to help athletes cheat on doping tests. Michael Rotich, the team's track and field manager, reportedly tried to bribe undercover journalists posing as coaches, offering to pay them in exchange for advance warning about drug tests.
August 24, 2016 - The International Weightlifting Federation reports that 15 Olympic weightlifters, including three Chinese gold medalists, have tested positive for illegal growth hormones and other banned substances in doping retests.
December 5, 2017 - The IOC announces that Russia is banned from the 2018 Winter Olympics in South Korea because of the country's "systematic manipulation" of anti-doping rules. However, Russian athletes who can prove that they are clean will be "invited" to compete. The ban is the most wide ranging punishment ever meted out by the IOS on a participating country. Russia's Olympic Committee is also ordered to reimburse the IOC $15 million for the cost of the investigation and to help establish a new Independent Testing Authority.
February 1, 2018 - The Court of Arbitration for Sport overturns lifetime bans on 28 Russian athletes accused of doping, making them eligible to compete in the 2018 Winter Olympics. In 11 additional cases, the athletes are suspended from participating in the 2018 Winter Olympics but they are cleared to compete in future events as the court rules that lifetime bans were not justified.
February 9-25, 2018 - The IOC allows 169 Russian athletes to participate in the 2018 Winter Olympics. They are not members of a Russian team. Instead, each participant is called an "Olympic athlete from Russia" (OAR) . They do not carry the Russian flag or wear uniforms that identify them as Russian. Their medals are not added to to country's Olympic medal count.
February 28, 2018 - The IOC announces that it is lifting the suspension on Russia participating in the Olympics. The country's status is restored after investigators confirm that there were no additional doping violations by Russian athletes who competed in the 2018 Winter Olympics.