Yew-berries are another poison that we should reject. There are a lot of drugs, herbs, poisons, and wines packed into the opening lines of this poem, and the speaker tells us forcefully to reject them all.
Quote #5
For shade to shade will come too drowsily, And drown the wakeful anguish of the soul. (9-10)
This is where the speaker tells us why we should reject all those drugs and poisons: it's because they will make us numb and "drowsy," and we should be awake and alert, even in our anguish.