When is a performance enhancement not?

In a recent post, I foreshadowed addressing the issue of painkilling injections.

Leigh Matthews reputedly commented, after the 2003 Grand Final, that there should be a shortage of painkilling injections, such were the injuries his team carried into the game. In fact, it is relatively common for players to either go into even home and away games with a jab or two, or to receive a jab during the course of the game.

Were a player to follow Warnie's lead and take a diuretic, he'd have a sanction applied if caught (even if only having a first strike recorded against his name). That diuretic could assist in losing some weight, or mask some other drug.

Were the player to use a steroid during training, he'd have a sanction applied. The steroid could assist in building muscle mass and strength, or in overcoming an injury.

The player getting a jab of local anaesthetic improves his performance because a specific pain is deadened to allow him to play as if the injury didn't exist.

I guess you'll see where I'm going here?

How is the anaesthetic any less performance-enhancing than the diuretic or the steroid?