When you ingest it your stomach acid will break most of it down causing a slow release in to bloodstream. But he broke the drug down into his mouth so it got absorbed immediately from his oral vessels into his blood. (And repeated use made jun build tolerance which the cop doesn’t have) as for jaeyoung they probably overdosed him on it.
there arent different reactions.. jun is a proper addict that consumes the drug on a daily basis— thats why he doesn't have withdrawals. the withdrawals are the effects of happy's issue with flashing lights(didn't you also notice jun wearing sun glasses?) . that investigatore abd happy's first reaction to getting drugged was the same.
Ok but I'm confused why the drugs seems to have different effects on people. Like why is this bitch Jun seems unaffected/affected differently by the Happy drug compared to others. It's effect on other people (Jaeyoung, the undercover officer for instance) is instant and lethal, but he's taking it like it's nothing. Or could it be he just got immune to it, sorta, due to taking it a lot? Hmm.