Final answer:
The important question to ask if a patient has stopped responding to an orally administered drug is if they have missed any doses.
Step-by-step explanation:
An important question to ask if a patient has previously responded to an orally administered drug but is no longer responding is Option C) 'Have you missed any doses?'. This question is crucial because non-compliance, or not taking the medication as prescribed, can lead to lack of response to the drug. Missing doses can result in suboptimal drug levels in the body, potentially compromising therapeutic efficacy.