probably because he was used to it?... like his father was a crazy dude so maby...
I think he already knew it was poisoned and probably vomited it out after eating it since he gave Bum salt water to induce vomiting when he got sick
some ppl say it was past the expiration date so it didn't have much effect, not on a big guy like Sangwoo (I think he must have thought that he could just go to the hospital if he got sick). Some say Sangwoo was testing Yoonbum and already changed the pills in the bottle with unharmful ones (I think in that case Yoonbum got sick because of the placebo effect).
I was also wondering why nothing happened to Sangwoo. But after reading this theory it makes sense. Thanks for sharing! (⌒▽⌒)
Ok i just noticed this, why didn't sangwoo got sick after eating the food with rat poison? O_O I saw he at at least a spoonfull of it and even said "ohh it's too salty"