There is no coherent argument to support the view that enhancing performance is unfair; if it were, We would ban coaching and training. Competition can be unfair if there is unequal access to particular enhancements, But equal access can be achieved more predictably by deregulation than by prohibition. That's Why
I'm assuming you mean performance enhancing drugs because having marijuana during a game would be ridiculous. If drugs were allowed athletes would have no reason to dedicate their lives to sport through hard work and self-discipline. Sport wouldn't be based on skill, Effort and strategy, It would be how well good of a doctor you have - anyone could get into professional sports leagues.