Final answer:
The FDA mandates that when refilling a prescription for diclofenac, a patient must be provided with a Medication Guide, which includes essential information on drug usage, side effects, and interactions.
Step-by-step explanation:
When Mr. Williams picks up a refill for his diclofenac prescription, the Food and Drug Administration (FDA) requires that he is provided with a Medication Guide. This is a special resource that comes with certain prescription drugs, and its purpose is to give patients detailed information about their medications. The Medication Guide includes information on the drug's uses, side effects, and potential interactions with other substances, which is particularly important when dealing with nonsteroidal anti-inflammatory drugs (NSAIDs) like diclofenac.