Steroids are a large class of bioactive chemicals that have caused scandals in baseball because of usage (and alleged usage) by a variety of players. While some sources claim that steroids have been used by major leaguers since the 1960s, very little attention in the press was paid to steroids until fairly recently.
Steroids contain a fused 4 ring, 17 carbon structure derived from cholesterol. As a group steroids have a wide range of biological functions, including androgenic (masculinizing), estrogenic (feminizing), and anti-inflamatory properties. Within the world of sports, the most significant and controversial class of steroids are the anabolic (muscle building) steroids, which are banned in most sports as illegal performance-enhancing drugs. When steroids are mentioned without qualification in a sports context, they invariably refer to anabolic steroids.
The issue came into the spotlight in the early 2000s with the BALCO controversy, which brought into question players such as Barry Bonds, Jason Giambi and Gary Sheffield. In leaked grand jury testimony, Giambi admitted that he had used steroids before contacting BALCO and that he had taken steroids provided by BALCO. Both Bonds and Sheffield denied using steroids, but prosecutors claimed that substances they admitted to using were "the cream" and "the clear", steroids that BALCO designed to be undetectable by conventional steroid tests. Since baseball instituted suspensions for first-time steroid use, there have been multiple suspensions of Major League and Minor League players. The biggest name suspension was Rafael Palmeiro, who had earlier claimed in testimony before Congress that he had never used steroids. Possibly the most attention brought to the issue came from Juiced, a 2005 book by Jose Canseco in which he admitted to using and reported that teammates Mark McGwire, Palmeiro and Ivan Rodriguez also were steroid users. A number of the players fingered by Canseco have since been confirmed as users, most notably Palmeiro (who failed a drug test in 2005) and McGwire (who came clean in 2010).
It is impossible to know exactly what percentage of major league players actually have used steroids or other performance-enhancing substances. Estimates of steroid use have varied wildly. Jose Canseco estimated that 85% of major leaguers were also using steroids. Ken Caminiti estimated that 50% of players were using steroids, but later retracted that claim and said that the number was lower. MLB's survey testing indicated a usage rate of 5-7%, but those numbers are unlikely to be reliable. They may be skewed by the ease of evading MLB's testing and the much publicized decision by some players to refuse testing and be counted as positive in order to drive up the positive rate as a way of forcing full-scale testing. The Mitchell Report, released in December of 2007, was an attempt by MLB to get a better grasp of how widespread the problem was, and how distribution networks functioned.
Which players are using steroids is another point of contention. While steroid investigations and speculation has focused on star sluggers, the players who have been caught by MLB's testing policy include pitchers and position players, stars and fringe players. Many of the minor league players who tested positive were fringe prospects.
Three prominent players got much attention for speaking out against steroid use while it was commonly occurring: Curt Schilling, Ken Griffey, and Frank Thomas. If history is a guide to us, in years to come these players who spoke out against steroid use will be honored, just as Christy Mathewson was honored for having spoken out against the tawdry practices in his era.
Steroids are not the only performance-enhancing drug, either. Pessimists have suggested that the reason that more players have not been caught using steroids is because the smart ones have started using human growth hormone, for which MLB long lacked an effective test, instead. This speculation was heightened by the arrest of Jason Grimsley on charges related to HGH, the admission by David Segui that he had also used HGH, and HGH related testimony from several of the players implicated in the BALCO case.
There was another wave of steroid cases early in 2015, when four pitchers - Arodys Vizcaino, David Rollins, Ervin Santana and Jenrry Mejia were handed 80-game suspensions in the span of two weeks. All four tested positive for Stanozolol, the steroid once made famous when sprinting champion Ben Johnson was stripped of his gold medal and world record in the 100-meter dash at the 1988 Seoul Olympics. Given its long history of abuse, it was a very simple product to detect, making people wonder why anyone would risk so much by taking a something that was certain to be captured in a drug test.
- Howard Bryant: Juicing the Game: Drugs, Power and the Fight for the Soul of Major League Baseball, Viking Press, New York, NY, 2005.
- Jose Canseco: Juiced: Wild Times, Rampant 'Roids, Smash Hits and How Baseball Got Big, William Morrow, Harper Collins Publishers, New York, NY, 2005.
- Will Carroll: The Juice: The Real Story of Baseball's Drug Problem, Ivan R, Dee, Publisher, Chicago, IL, 2005.
- Tim Elfrink and Gus Garcia-Roberts: Blood Sport: Alex Rodriguez, Biogenesis and the Quest to End Baseball's Steroid Era, Dutton, Penguin Books, New York, NY, 2014. ISBN 978-0525954637
- Mark Fainaru-Wada and Lance Williams;: Game of Shadows: Barry Bonds, BALCO, and the Steroid Scandal that Rocked Professional Sports, Gotham Books, New York, NY, 2006.
- Alan M. Nathan: "The Possible Effect of Steroids on Home-Run Production", in The Baseball Research Journal, SABR, Volume 38, Number 1 (Summer 2009), pp. 36-38.
- Jorge L. Ortiz: "Why the positive steroid tests? MLB wants to find out", USA Today' April 12, 2015. 
- Kirk Radomski and David Fisher: Bases Loaded: The Inside Story of the Steroid Era in Baseball by the Central Figure in the Mitchell Report, Hudson Street Press, New York, NY, 2009. ISBN 1594630569