Zoro took Hakai so he would have to take the Mink medicine for some reason.
Chances are, the reason is that so Zoro can be limited in the next arc to prove how "tough" Onigashima was on the crew. Everyone else will be fine but Oda likes making Zoro the guinea pig for severe injuries so he'll be the one to suffer. He'll be 100% the arc after though, so there's that.