Sorry no proof. Sasuke took Oros powers it's his now. Sasuke defeated him and took his powers no differnet to how Sasori does things. Oro made Sasuke immune to poisens by exposing it to him. Thats just being a bad/good teacher. It's called training
What would be the point of Kishi putting it in the manga if there wasn't any proof behind it. There is more proof to suggest that he would and did, than him saying "I know these will make me stronger, but I would rather train than take these drugs". If he thinks he will become more powerful he does it.
So by that logic every time naruto took control of the fox's power it was considered his own power, even before he tamed Kurama?
It was never shown that Sasuke took any drugs while training under Orochimaru, and for your sprinter analogy it depends what you define as drugs, would you say vitamin pills or protein supplements as drugs?
But it was heavily implied. Kishi didn't have to put that detail in, it wouldn't have affected the overall story much, but he didn't. Also considering the character there is no reason he wouldn't
- He stated on many, many occasions he was willing to do anything to gain power
- He had previously taken Orochimaru's mind awakening pill, a drug that improved his cursed seal, even though he was told he will possibly die
- He trained under Oro with Kabuto supervising, both of which are experts in ability enhancing drugs
He had motive, a past history of it, and the tools necessary to obtain these drugs. He definitely took them. Also the drug-use was referred to in response to Sasuke's increased ninja ability. I compared them the steroids as these are also performance enhancing drugs.