As a law-professor, I haven't been tempted to take anabolic steroids, HGH or other performance-enhancing drugs (other than caffeine!). Thus, my knowledge of their effects is admittedly amateurish, based as it is on casual sports fandom, internet surfing, and watching the movie Better, Stronger, Faster. From what I've been able to figure out, depending on the drug, the benefits decay at different rates over time. Thus, a baseball player who took drugs to build muscle mass in, say, 2001, 2002 and 2003, and then stopped, would see the most benefit during those years, but depending on his fitness regime to maintain that mass and the particular drug, would continue to benefit in later years. For this reason, it's hard to take seriously the claim that use of PEDs taints performances during the period when those drugs are taken but not thereafter---a claim being made at least tacitly by A-Rod and others.
An analogy might be useful here. Consider the case of Oscar Pistorius, the South African runner who was originally forbidden from international competition because his artificial limbs were thought to give him an advantage over runners with intact legs and feet. The Court of Arbitration for Sport later overturned that ruling, although Pistorius failed to run a fast enough time to qualify for the Beijing Olympics. But suppose a clearer case. Suppose that a mediocre baseball pitcher named Lefty undergoes surgery and has installed in his arm socket a mechanical device that enables him to throw a 150 mph fastball with movement and control. Suppose further the surgery does not violate any rule of baseball. When the results of the surgery become apparent, however, Major League Baseball decides to ban this and related surgical improvements to players. Even though Lefty didn't violate any rules of baseball by having the surgery, it would be perfectly sensible to apply the rule to him because he derives a continuing advantage from it.
So why wouldn't we apply the same principle to past use of PEDs? Indeed, baseball players who used PEDs when baseball did not ban them were still breaking the law because these were controlled substances obtained illicitly, so they have less cause to complain than does Lefty, because Lefty's surgery was legal. The real problem here is simply one of enforcement: Because various PEDs decay over time, we don't have a good way now to figure out who is deriving continuing benefits from past PED use. Records we do have---such as the samples taken before baseball banned PEDs---were obtained only on a promise that they would not be used to discipline players, and thus cannot be used because of a kind of exclusionary rule.
But to repeat, those are practical problems about enforcement, not reasons in principle why athletes who continue to derive benefits from PEDs are somehow entitled to those benefits because they have stopped taking the drugs. To the extent that the PEDs work like Lefty's artificial super-arm, they confer an ongoing unfair advantage.
Posted by Mike Dorf