An interesting question. Well, the first thing I'd do is to challenge the charge. Murder is not just an issue of whether someone killed another person. Murder involves motive. The other side would have to prove that the apothecary intended to kill Romeo. Your job as defender would then be to develop innocence in this area. I'd ask questions establishing a lack of motive. How did the apothecary feel towards Romeo? Did he hold any grudge or ill will? Etc.
There's also the question of guilt in the sense of responsibility. By that I mean, how is the apothecary responsible for what Romeo does with a product? If you are feeling ambitious, you could call someone else who sold daggers and have them testify that they are not charged with murder when someone defends him or her self.
I'd also call the Friar. The Friar distributed drugs as well, and had a hand in the chain of events. Establish his guilt.