In the Shibusawa gem storage Just seconds before you see someone trying to stab Dazai but you stop them so you get injured. It's all over, you are taken to the hospital because the stab from that person not only causes injury but it is also poisonous. You wake up with Dazai by your side still wearing his neat suit, and he suddenly kisses you out of the blue. That's the antidote. He says after giving you the antidote by kissing you.