Greenies, eh? I believe they are a banned substance as well and they test for them. The MLB and the MLBPA rarely agree, but on this they might have to work something out.
Here's a list thats been going around, can't vouch for the veracity:
Jose Guillen, Jay Gibbons, Juan Gonzalez, Clay Hensley, Jerry Hairston, Felix Heredia, Jr., Darren Holmes, Wally Joyner, Darryl Kile, Matt Lawton, Raul Mondesi, Mark McGwire, Guillermo Mota, Robert Machado, Damian Moss, Abraham Nunez, Trot Nixon, Jose Offerman, Andy Pettitte, Mark Prior, Neifi Perez, Rafael Palmiero, Albert Pujols, Brian Roberts, Juan Rincon, John Rocker, Pudge Rodriguez, Sammy Sosa, Scott Schoenweiis, David Segui, Alex Sanchez, Gary Sheffield, Miguel Tejada, Julian Tavarez,Fernando Tatis, Maurice Vaughn, IJason Varitek, Ismael Valdez, Matt Williams and Kerry Wood, Brady Anderson, Manny Alexander, Rick Ankiel, Jeff Bagwell, Barry Bonds, Aaron Boone, Rafaeil Bettancourt, Bret Boone, Milton Bradley, David Bell, Dante Bichette, Albert Belle, Paul Byrd, Wil Cordero, Ken Caminiti, Mike Cameron, Ramon Castro, Jose and Ozzie Canseco, Roger Clemens, Paxton Crawford, Wilson Delgado, Lenny Dykstra, Johnny Damon, Carl Everett, Kyle Farnsoworth, Ryan Franklin, Troy Glaus, Rich Garces, Jason Grimsley, Troy Glaus, Juan Gonzalez, Eric Gagne, Nomar Garciaparra, Jeremy Giambi
Apparently the real list will contain more pitchers than position players (not surprised).