it will work too well, it wont work at all, romeo wont show up or just wont care about her "death" . Or being alone with all the bodies, continuing their lives together.
She fears that either it might not work and she'll have to marry Paris the next morning, and that it might be a poison and she might die.
"What if this mixture do not work at all?" (Act 4, Scene 3, 21.)
"What if it be a poison, which the friar/ Subtly hath minister'd to have me dead… (Act 4, Scene 3, 24-25)