Final answer:
The nurse should instruct the client to avoid consuming grapefruit or grapefruit juice when starting a new prescription for extended-release verapamil.
Step-by-step explanation:
The nurse should include the instruction to avoid consuming grapefruit or grapefruit juice when teaching a client who is starting a new prescription for extended-release verapamil. Grapefruit and grapefruit juice can interact with verapamil and increase the levels of the medication in the body, leading to potential side effects and adverse reactions. Therefore, it is important for the client to avoid consuming grapefruit or grapefruit juice while taking verapamil.