Herbal medicine has been used for centuries and has undergone a renaissance in recent years, as people seek more holistic ways of preventing disease. Many modern drugs are based on herbal medicine; for example, the invention of aspirin came from the use of willow bark for pain and its active ingredient, salicylic acid, is derived from the willow tree.

Friar Laurence is a learned holy man who has vast stores of herbal knowledge, which was common for monks in Shakespeare's time. Prior to the development of western medicine, herbs were widely used for healing in cultures around the world. The mystery novels and the television series Cadfael portray a medieval monk who solves murders and often relies upon his knowledge of the natural world to do so.

The Friar's herbal knowledge is displayed in his monologue when he is in his cell, thinking he is alone. But Romeo enters at a crucial point and overhears him, which is the moment that foreshadows Romeo seeking the Friar's help in finding a way to marry Juliet, which causes Romeo's death due to his wrongful belief that Juliet is dead.

In his speech, the Friar compares the benefits and dangers of herbs to human nature:  

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, stays 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.

In this speech, we see that the predominant nature of plants and humans comes down to a binary view of human nature, where qualities can change based upon context and other factors. The idea that a plant can contain properties of both poison and medicine is being compared to the human qualities of "grace and rude will." 

In act two, scene three of Romeo and Juliet, we meet Friar Lawrence, who carries a basket of flowers, weeds, and herbs, and speaks to himself about the special properties that they possess. He doesn't identify any specific powers that the herbs possess, but he says that they all have the power to do good or harm, depending on how much of each is used. As he puts it, "Virtue itself turns vice, being misapplied."

The idea here is that a medicine becomes a poison if administered in the wrong dose and vice versa. This is ironic because the help that Friar Lawrence gives to Romeo and Juliet, although meant kindly, becomes, by the end of the play, poisonous. Friar Lawrence tragically fails to apply his learning about herbs, weeds, and flowers, to his own actions.