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Sonnen says Jon Jones popped for estrogen blockers, here's what that means

As Chael Sonnen claims Jon Jones tested positive for estrogen blockers in his UFC 200 out-of-competition drug test. Iain Kidd examines why an athlete would want to take estrogen blockers in the first place.

Joshua Dahl-USA TODAY Sports

Note: The below is presented for informational purposes and should not be treated as an allegation or confirmation that Jon Jones has taken any specific drug or substance. This is intended to be a basic introduction into estrogen blockers as prohibited substances and is not an exhaustive explanation of their mechanisms of action.

Fox Sports' Damon Martin reported on Twitter that Chael Sonnen made a claim on Joe Rogan's podcast that Jon Jones tested positive for two estrogen blockers (which led to his removal from UFC 200). saw a lot of confusion and misinformation circle about estrogen blockers, so I decided to clear things up a little.

Editor's note: On Friday, Rashad Evans stated in this CBS Sports video that Jones' camp told him the positive result was for estrogen blockers.  (H/T Steven Marrocco of MMAjunkie)

Estrogen blockers aren’t, by themselves, a performance enhancing drug in the way we normally think of it. An estrogen blocker won’t make you any stronger or faster or give you better cardio. Despite that, estrogen blockers are extremely popular supplements. Why? Because they’re useful as "post-cycle therapy."

There is some evidence that certain estrogen blockers, such as "Clomid," may also increase testosterone production in healthy men. Testosterone is the primary steroid hormone, and increasing testosterone levels would increase muscle mass and athletic performance.

A "cycle" in this instance refers to a cycle of steroids, prohormones or other exogenous anabolic agent. When you increase the amount of testosterone in your body, it undergoes a process called aromatization, which converts a portion of it into the female sex hormone estriadol, which is the main active estrogen, and is often referred to simply as estrogen as a result.

While the body requires a certain amount of estrogen to function effectively, too much estrogen in a male will have undesirable side effects. The primary undesirable side effects would be increased subcutaneous fat gain, and potential gynecomastia, or breast enlargement.

As a result, most cycles of steroids or similar PEDs are accompanied by and followed with compounds designed to reduce estrogen levels and/or reduce how they affect breast tissue and the HPTA axis. There are various means of doing so, with the most common being to take a selective estrogen receptor modulator (SERM) like breast cancer drug Tamoxifen or fertility drug Clomid, or an aromatase inhibiting compound. SERMs and aromatase inhibitors are banned in and out of competition by USADA, as are other anti-estrogenic substances.

Selective estrogen receptor modulators, aromatase inhibitors, and other anti-estrogenic substances ensure estrogen levels stay low and/or help to prevent the aromatization of testosterone into estrogen both while "on cycle" and after a cycle, when the body’s hormone levels are out of balance. SERMs also help to minimize the decrease in luteinizing hormone production which can be caused by high levels of estrogen. Luteinizing hormone is vital in the production of testosterone. 

There is a belief among some PED users that lowering estrogen levels also triggers the body to naturally increase testosterone production. If true, this would be useful, as after a cycle of steroids the body's natural testosterone production is diminished and does not recover immediately. The scientific basis for this particular claim seems to be rather limited. Update: There are studies which suggest natural testosterone production can be increased by certain estrogen blockers in individuals who present with low testosterone. Thanks to twitter user Joseph Koerner for highlighting this study.

Several "testosterone booster" supplements designed for use as post-cycle therapy also contain drugs designed to reduce estrogen levels because of this belief.
The FDA has initiated recalls on supplements illegally containing aromatase inhibitors in the past. These and other banned estrogen reducing/controlling substances are often taken by PED users in order to avoid many of the negative side effects that an unnaturally high level of testosterone would otherwise bring.