Lilly of the valley + risin, slight confusion?

I’ve watched the series all the way through at least 5 times and for some reason, I just can’t grasp this storyline.

I understand Brock was poisoned by the plant but later in season 5, why did Heul pickpocket the risin cigarette from Jesse? And then Jesse freaking out when he realized he didn’t have it? I’m a little confused by that entire storyline.

Can someone please break it down for me, thank you so much!