Medea uses her magical powers even before the actual action of the play begins. In the myth of Jason and the Argonauts, we hear that Jason was able to capture the golden fleece from the dragon that protected because he had magical help from Medea. This magic helped him defeat all of the challenges put before by King Aeetes, Medea's father. She knows that she has betrayed her father and makes Jason promise to take her with him as his wife once he takes the fleece.
Next Medea uses her magic to kill Jason's uncle, and then create a potion that restored his own father's health and youth so that could once again rule his kingdom.
After they are banished to Corinth, things remain calm for many years, but once Jason decides to set aside Medea and their children in order to marry the princess of Corinth, Medea's wrath returns. She promises the use of magic to King Aegues of Athens for him to have children, in exchange for future safe haven. She uses magical poisons to kill and decimate the body of the princess, and she uses magic to produce a flying chariot that whisks her away from Corinth and the wrath of Jason and the King's men who would have her killed as punishment for her crimes.
Medea is a vengeful and dangerous women who should not be tested. She will use whatever skills and knowledge she posseses in order to do what she determines she needs to do.