What doesn't kill you makes you stronger—unless it puts you into a near-death state that fools your husband. The point is, poison (and medicine) are a big deal in Romeo and Juliet. Like love and hate, the difference between them is pretty slim.
Before Romeo and Juliet take their lives, Friar Laurence, who's big into herbal medicine, shows Romeo a flower and makes a cryptic statement that seems to echo throughout the play:
Within the infant rind of this weak flower
Poison hath residence and medicine power:
For this, being smelt, with that part cheers each part;
Being tasted, slays all senses with the heart.
Two such opposed kings encamp them still
In man as well as herbs, grace and rude will;
And where the worser is predominant,
Full soon the canker death eats up that plant. (2.2.1)
Friar Laurence suggests that, depending on how it's used, a flower can be healing (because it's aromatic) or poisonous (if it's orally ingested). The Friar also muses that people are a lot like the flower he holds in his hand—being full of both "grace" and "rude will," human beings also have the capacity to be good or deadly, depending on whether or not "rude will" takes over.
We can't help but notice that Friar Laurence's observations speak directly to the play's tragedy—Romeo and Juliet's love turns deadly when it's "poisoned" by their family's hateful feud. At the same time, their love also has the capacity to heal, which becomes evident when their parents decide to reconcile at the play's end.