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What is the difference between "legal" use of performance enhancing drugs and scientific based performance enhancing training techniques?
From an athlete welfare point of view, nothing, but you might as well ask what the difference is between racing le Tour on safety bicycles and doing it on recumbents. Sports rules are arbitrary, and in the case of most sports there is an arbitrary distinction between physiological interventions which have the same costs and benefits to the athlete where one is considered natural and the other is considered synthetic. The line is drawn by moral philosophers drawing from classical antiquity and the Abrahamic faiths. There is an equally arbitrary distinction between "food" and "drugs", not just in sport but in society, one which varies between cultures. If global sports had originally been codified in China, the rules might be different.
What is the difference between "legal" use of performance enhancing drugs and scientific based performance enhancing training techniques?
Both are often without regard to health. I have known more than a share of athletes-- who did not use drugs-- who were physically damaged by their sports-- athlete’s heart and heart arrhythmias being one of the most common of the lot.