A 38-year-old male with severe migraine headaches is prescribed a new medication that acts as an agonist at a specific serotonin receptor. The medication provides some relief, but the patient continues to experience debilitating headaches. The physician decides to add a second medication to enhance the antimigraine effect. The graph above shows the dose-response curve for the initial antimigraine medication alone (green line) and the dose-response curve for the initial antimigraine medication in the presence of the second medication (yellow line).