Doping

Is there anything you can actually use to boost performance, but that is legal, over the counter and does not have any serious side effects?
I'm currently using beet juice, it's helping a bit but it's best to avoid the toilet.

Carbohydrates are proven performance enhancers for extended exercise.

Caffeine is a proven ergogenic aid but may have side effects for some people.

There's evidence for beetroot juice and creatine being legal performance enhancers for different stuff.

Most other stuff is unproven or illegal.