Best Medicine allows Harley Quinn to support and sustain her lane partner. Harley Quinn can use it on herself.
Puddin' Time! is a strong crowd control ability, but due to its conic shape, it is generally harder to hit if you target is too close.
Since Back Off is so crucial to her ability as a controller and it is melee range, Harley Quinn needs to build a little more durable than most controllers. When you are already fairly close to your target, knock them up with Puddin' Time!, get ahead of them with BFFs' speed boost and knock them backwards into your team with Back Off.