Many of William Shakespeare's plays were based on earlier imaginative or historical texts, a practice common in his period. For example, Julius Caesar dramatizes an essay by Plutarch. In the case of Romeo and Juliet, the theme of lovers whose parents disapprove of their relationship is a staple of Greek New Comedy and Roman Comedy. The specific plot details derive from "Hystoria di due nobili Amanti" by Luigi da Porto, adapted into English as "The Tragicall Historye of Romeus and Juliet" by Arthur Brooke. While many elements of Shakespeare's plays reflect the general state of cultural and scientific knowledge of his period, there is no evidence of any particularly significant interest in alchemy beyond a general lay knowledge.
Friar Laurence seems to be one of a long line of friars including Robert Greene's Friar Bacon and Friar Bungay, which was first performed as early as 1592, and printed in 1594. With Shakespeare's friar, Brooke's antifraternal tradition gives way to an altogether different vision. What's more, it seems that the attitude of Friar Laurence to plants and medecines in Act II is analogous to Paracelsan physicians who thought that poison could turn out to be an antidote to a malady and cure instead of killing. This is in an accordance with The notion of concordia discors, which is omnipresent inShakespeare's plays. Any further information about this topic is welcolme...especially as far as Roger Bacon, the Franciscan friar is concerned.