Colts' Robert Mathis Admits to Drug Use

The defensive end claims he took the performance enhancer to increase his fertility.
3:14 | 05/20/14

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Transcript for Colts' Robert Mathis Admits to Drug Use
Now, to one of the NFL's top players fighting back after getting suspended for failing a drug test. Robert Mathis took a banned substance. But he claims he did it to enhance his fertility. Alex Perez has the story. Reporter: Robert Mathis is defending himself. He's an all-pro player, who helped make the colts' 2006 super bowl champs. But Mathis has been suspended four games testing positive for clomid. There's not one bit of evidence that Robert used this for anything but fertility. Reporter: Clomid is banned by the NFL because it can be used to help enhance a player's performance. But Mathis' agent says he took the drug to help him and his wife conceive. The couple has twin boys and a daughter. But the couple wants to give his ailing mother a fourth grandchild. The league should have looked at this case and said, Robert, you made a mistake. Reporter: But the NFL is firing back. Saying the player had an opportunity to go to several sources that he could have contacted. All of which would have advised him not to take the substance. The failure to do so as consequences. Clomid is fda-approved. It can raise the testosterone levels. It can cause muscle growth and increase in muscle size and strength. Reporter: The colts' coach, standing standing behind his player. Mathis' wife is now pregnant. And in a statement, Mathis admits he made a mistake, not checking with the league before beginning his treatment. And has accepted his suspension. For "Good morning America," Alex Perez, ABC news, Chicago. We're going to bring in ABC's chief legal affairs anchor, Dan Abrams. Do you agree with the stance of the NFL? I don't think they have a choice here. He could have gone to the NFL first. He didn't. The NFL is saying, this wasn't an fda-approved drug for malar iftivety. It was banned by the NFL. Isn't it clear when it comes to the rules where that line is? And what you put into your body is the responsibility. Does the doctor have responsibility in this case? Now, you have Mathis pointing the finger at his own doctor. He cleared me. I should have cross-checked it. The doctor being careful. A lot of patients don't hear everything when you go in there. Look, you can't point the finger at your doctor in a case like this and say, he told me it would be okay. You're an NFL player. You know the rules. You can't say, his fault. And the doctor even said he wasn't told he was an NFL player. You would open with that. If you -- You have an opportunity to take a lot of different things. But you take them to the team, if you're not sure. You show it to your trainers. Saying the doctor checked this out. And anything that boosts your performance is a violation of the -- Ask first. That's the key. He's going to stand by the four-game suspension. Thank you very much.

This transcript has been automatically generated and may not be 100% accurate.

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