i'll take a guess that the only way for the effects of the drug would wear off is for Shin to admit his feelings for Wolfgang--and tell him or anyone?
The guy who gave him the truth serum asked back then, "In this moment, what is the one thing you think you shouldn't divulge?"
So even now, Shin is fighting the magic of that serum and it's causing this adverse reaction.
IN SUMMARY (≧∀≦):
Magic truth serum + Shin not divulging his feelings --> he can't get the magic out of his system --> adverse reaction
I've watched the trailer just now. I guess it happens with a lot of adaptations, b-but--the achingly subtle yet beautiful portrayal of the romance in the mahua... If the author was consulted, then that's fine but everything in the manhua was perfect as they were for me
nevertheless, i'm so excited to watch it when it finally airs!!
EDIT: The drama's a bit different but enjoyable so far! Wish they'd hold off on Sun knowing Changge's female though. Still, it's fun to watch.