>>532583453
Shadria cuddles
Shadria hugs
Shadria handholding
Shadria kisses
>>532583965
Well, to start out with the most obvious thing, Shadow's memory/mind has been manipulated against his will several times over the course of the series. Both Gerald and Black Doom wanted to make him forget what he valued most. I can't imagine repeated mental alterations were a fun time, so I believe that Shadow prefers to think clearly without any outside forces altering the way he perceives things.
He's lost his memory before, and drugs often alter peoples' memories.
Hoshino clarified once that Shadow does not drink alcohol.
In SA2 he's incredibly alert and wary, his idle has him looking around warily and saying danger is all around him. He doesn't seem to like fog or mountains.
He's also described as "pure" both in-universe and out of universe.
That and we don't know how much they tested on him, it's possible they'd drug him back on the ARK to see what it would do.
It's not a lot, but it's enough to make me think that he'd avoid drugs.