Within the infant rind of this small flower
Poison hath residence and medicine power.
For this, being smelt, with that part cheers each part;
Being tasted, stays all senses with the heart.
Two such opposèd kings encamp them still,
In man as well as herbs—grace and rude will. (2.3.23–28)

Friar Lawrence talks about a plant that has both medicinal power, if smelled, and the poisonous power to kill, if eaten. He claims that this dual nature is present in everything, including people, and likens the poison in the plant to the “rude will” found in men. Poison therefore symbolizes the dark side of human nature. This type of poison will play a major role in the events that follow, ultimately leading to the deaths of the two young lovers, Romeo and Juliet.

Hath Romeo slain himself? Say thou but “ay,”
And that bare vowel I shall poison more
Than the death-darting eye of cockatrice. (3.2.46–48)

Juliet says that if Romeo has killed himself, she will become figuratively like poison—angry, dangerous, harmful—and even more poisonous than a snake. Poison is a symbol of the dark side of humans.

There is thy gold, worse poison to men’s souls,
Doing more murder in this loathsome world,
Than these poor compounds that thou mayst not sell.
I sell thee poison. Thou hast sold me none. (5.1.84–87)

Romeo buys poison from the poor apothecary, which he plans to take to Juliet’s tomb and then drink himself. Here, he claims that the money he uses to pay for the poison is a type of poison itself, as money has been the cause of more deaths than the actual poison he bought ever has. Poison symbolizes anything that is evil and harmful.

(kisses JULIET,takes out the poison)
Come, bitter conduct, come, unsavoury guide.
Thou desperate pilot, now at once run on
The dashing rocks thy seasick, weary bark.
Here’s to my love! (drinks the poison) (5.3.125–129)

Poison is harmful and evil, but here Romeo also calls it a way, a guide, and a pilot—three things that will bring him relief. Here, poison symbolizes his escape from this world, the cruel world that took Juliet from him, the empty world with which he’s grown tired.