Perfomance enhancing drugs

Just something that has always bugged me. For example Nadal is having a series of cortisone injections to get him through Wimbledon. How is this no enhancing his performance? Because he'd definetly lay better with the injection than without.

Difference being that it won't increase his potential max performance.

If you view Nadal's normal performance as being 100%, the cortisone means he can play through the injury - with some limitations as it's not a cureall, and doesn't guarantee the injury won't get worse. His performance still won't exceed his normal high performance.

Performance enhancing drugs on the other hand, would lead to the normal performance levels being increased.

It's a good question this. It seems odd that a cortisone injection is OK but that Manchester City player was banned because he took water tablets????

I know it all comes down to the ingredients used in each case, but cortisone is an an Adrenaline hormone based drug so you would think it's use was questionable at best.