Chicago Bears wide receiver Brandon Marshall swears he never took any performance enhancing drugs. But he tells the AP he's heard of "some crazy stuff," like "guys using Viagra." He made the comments when asked about amphetamine use—many players have been testing positive for Adderall and other stimulants lately. Marshall's not sure how widespread that problem is, but he will say:

It's such a competitive league and guys try anything just to get that edge. I've heard of guys using Viagra, seriously, because the blood, it's supposed to thin ... It's kind of scary with some of these chemicals that are in some of these things, so you have to be careful.

Actually, many athletes don't have to be careful about Viagra, technically. The World Anti-Doping Agency didn't list it as an officially banned substance at the Beijing Olympics, for instance. NFL rules stipulate that all medication—over the counter or otherwise—has to be approved by training rooms. "Some of it's fair, but some of it kind of puts you in a tough spot as a professional athlete," says Marshall. "Because the only thing you'll see is getting busted for PEDs, but it could be something over the counter for a little sickness. You just have to be on top of your Ps and Qs." 

Athletes take Viagra in hopes that it will increase bloodflow and thus oxygen delivery. But many scientists are skeptical about the pill's purported performance enhancing effects. It would be "extremely difficult, if not impossible" to prove Viagra boosts athletic performance, UCLA's  Anthony Butch told The New York Times' Jeré Longman back in 2008.