From what I've heard guys in the know say (like Victor Conte), as tests get more advanced, so too does the ways to cheat them. Holyfield only got caught because they found his name and reciepts with his name when they raided a business that was providing pro athletes with illegal steroids. He never failed a drug test once! He just had the misfortune of buying from a business that got busted hahah (same with Shane Mosley and a few others)

Guys are still getting busted steady in boxing and MMA, so that says to me that there are ways to get around it. If drug testing was foolproof, nobody would be doing them. And the chance of getting away with it has to be pretty good if guys are willing to risk the shit storm that comes with getting caught.

Not that I'm saying Tyson did them for sure... I'm just not willing to say that Holyfield was juicing and all his opponents were clean haha