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Tamoxifen

Marlon Byrd of the Red Sox bats against the Oakland Athletics at Fenway on May 2, 2012, shortly before he received a 50-game suspension for taking the banned substance tamoxifen.

J Rogash/Getty Images

In June 2012, outfielder Marlon Byrd faced a 50-game suspension after testing positive for tamoxifen, a substance banned from Major League Baseball and many other sports. Why would an athlete want to take a drug normally used by breast-cancer patients? The answer lies in some interesting biochemistry.

Many breast cancers have receptors for estrogen, a hormone that promotes the development and maintenance of female characteristics of the body. When estrogen molecules fit into these receptors, like a key fitting into a lock, the malignant cells become activated. Tamoxifen blocks these estrogen receptors, interfering with the cancer's ability to grow and develop. This is why scientists refer to tamoxifen as an anti-estrogenic agent.

Now let's turn our attention to a home-run slugger taking steroid injections -- usually synthetic testosterone -- to grow his muscles. Large doses of the male hormone cause the body to produce additional estrogen. This in turn can result in enlarged breasts, a feature that most power hitters find unappealing. To counteract the effects of estrogen and mask their steroid use, these players may opt to take tamoxifen. That means anti-estrogens don't really enhance performance, but, because they alleviate symptoms of PEDs, they appear on the World Anti-Doping Agency's list of more than 200 banned substances and methods.

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