One of the devices Shakespeare uses to make the scene when Romeo buys poison from the apothecary affective, Act 5, Scene 1, is irony.
Romeo asks the apothecary for a poison that will speedily take affect and "disperse itself through all the veins / That the life-weary taker may fall dead," plus "violently" suck the breath from a man (V.i.64-65, 67). The apothecary replies that he certainly has such drugs, but that they are illegal in Mantua and that the punishment is death. The irony is that, in Verona, Prince Escalus has recently made endangering a person's life through fighting illegal and punishable by death. Romeo was banished from Verona for breaking that law by slaying Tybalt. Now, ironically, he wants to break a similar law by taking his own life, thereby jeopardizing the life of the apothecary.
Further irony can be seen in Romeo's response to the apothecary's reluctance to give Romeo the poison. Romeo argues that it's very obvious the apothecary is very poor and that "oppression starveth in thine eyes" (73). He further states, "The world is not thy friend, nor the world's law ... Then be not poor, but break it and take [the great sum of money Romeo has offered him]" (75-77). The irony is that these lines apply more to Romeo than to the apothecary. While it's true that the world is doing nothing for the apothecary's poverty, it is Romeo who views the world and its laws as enemies to himself as they separated him from Juliet due to his banishment.
These instances of irony help us to see exactly what Romeo is suffering and exactly what he is about to do, adding to the drama of the situation. In addition, this scene is an example of dramatic irony because we know that Juliet is not really dead, and that Romeo is about to needlessly kill himself due to the suffering he has experienced from the world and its laws, further adding to the drama.